Rear end bucking during 1st & 2nd gear crawling

So this is baffling me, and hopefully someone will know exactly what I am talking about.

1998 XJ, 4.0L, 4x4, Manual 5sp, Pretty much bone stock, 138k miles

When I am taking off in 1st or just switch into 2nd gear I will get a violent bucking from the rear end. Almost like it is wanting to dry hop. I can make it do it on command by going under 5mph in 1st gear any lightly giving it gas to load up the rear end. If I do any sudden input of gas or release pressure and resume pressure it will start bucking 3-4 times until the jeep "unloads" itself from the acceleration. This is magnified if I hit a bump and it causes my foot to bump or pull off the throttle.

I have already replaced (today) the Transmission mount and the drivers side motor mount. Both of which were old, but not completely gone, however the problem is the exact same. :helpme:

I can grab the driveshaft and pull it from side to side without noticing hardly any play in the front or rear U-Joints. If I rotate the driveshaft back/forth I hear a slight clicking in the rear end, but I think that is just the pinion gears rotating back and forth.

Otherwise the jeep drives exceptionally smooth. No vibrations or noises at any rpm or speed.

Any thoughts? Could it be rear leaf springs worn out? Shackles/Bushings? Struts? Rear end? T-Case?

Help would be greatly appreciated.

I had that happen once, the collapsible bushing in my 8.25 collapsed. My guess is from jamming the drive shaft in some odd way, from towing a way overloaded tailor. The pinion got way loose. I caught it before it did any serious damage and a new collapsible bushing was all I needed.

May be your issue or it may not, but check the rear yoke and pinion.

My 88 MJ does something very similar to the op. Haven't figured mine out yet

In regards to the colapsable bushing....did you replace this yourself? Doesnt this require removing the pinion, carrier, and re-shimming/backlash of the entire rear end? This might be a little over my head. I have replaced axles and removed c-clips etc. but did not mess with the carrier or pinion before.

How would you suggest I check to see if it is the colapsable bushing?

Also whats the best way to check the rear yoke? I havent noticed any play in it, but maybe there is some other thing I am missing.

It has been along time ago, but all I did was replace the collapsible bushing and torque to specification ( you need a giant heavy duty torque wrench and a hefty guy to do this). I did check for pre-load and did a contact pattern check. It was a low mileage rear end, the bearings were still good. I marked the carrier bearing adjuster and put everything back together just like I found it.

Mine was easy to spot after I put the rear up on jack stands, put it in neutral and turned the drive shaft by hand. My yoke and pinion kind of flopped around some.

The way I test the pinion is to slap the side of the yoke with the palm of my hand and feel/listen/look for any play. I sometime take the tires and brake drums off and turn the yoke by hand, listen and feel for anything that rumbles

The collapsible bushing may not be your issue. If the yoke is in there solid with no play, you may want to pop the cover and do a visual on the gears. If one tooth is broken this may be causing your problems and may eventually destroy just about all your gears.

I also pry up on the carrier to check for bearing play. The carrier is heavy, you may not notice carrier bearing play due to the weight. If you pry up on the bottom of the whole carrier assembly you can see/feel/hear it move up and down. Though worn carrier bearings tend to get noisy and most times even noisier in reverse.

We may be on the wrong track anyway, have you checked the U-bolts holding the axle to the springs. People sometimes over torque them, once over torqued they tend to stretch pretty easy. I saw this just a few months ago on a buddies rig.

If you pop the cover, save the oil and run it through a paint filter (or a coffee filter or even a rag) and see what kind of metal you have in there. If you heat up the oil it works a lot better.

U-joint or axle splines are also a possibility.

I've never had the chain pop (jump the sprocket) on a 231 or a 242, but sure have in other transfer cases. It usually does just like you described, it pops and bucks when you accelerate. I really don''t know if you want to try this or not, but I've nudged up against a tree and gave it some gas and the chain jumped the sprocket. I then knew it was likely the transfer. There is always the chance you'll bend the bumper or grenade something doing this.

Thanks for all the input. This morning I threw a jack under the rear end, put the tranny (and tcase) in Neutral so everything would spin freely.

I grabbed, tugged, and pushed on the yoke/pinion/driveshaft and could not create any movement that was not a normal rotation. I pulled on the wheels to try and generate play in the axle shafts or c-clips and it all felt tight as well.

If I held the tire and spun the driveshaft there was maybe a 1/4" rotation before the gears would bind. Is this normal? By rocking the driveshaft back and forth (IE: forward to reverse) it would give me a 1/4" of play before engaging. I just thought this was the planatary gears turning? I still think it might be suspension related, but everything felt tight. Could worn out struts and leaf springs cause the axle to try and drive under the car so to speak and create a bouncing sensation?

Maybe my problem wasn't explained correctly. It doesn't seem to be a metal on metal violent bucking....but more of a repetitive lurching from the back end. Only under very specific conditions. If I let out the clutch to hard it will start to lurch/rocking from rear end.

Pull the driveshaft and lube the slip yoke where it goes into the t-case.

Any particular grease for the slip yoke or just a heavy duty bearing grease?

Thanks for all the input everybody....and if any other suggestions come up, please do not hesitate to give them.

Rear end bucking during 1st & 2nd gear crawling

Clutch/driveline issue? Fuel delivery problem? Sounds like it could be anything. Pop the rear driveshaft off and front wheel drive it around the block see if it goes away?

Mine XJ does something along these lines and figured it was due to the soft OME rear lift springs and the amount of exposed spline on the slip yoke. I have to push in the clutch to get it to stop rocking.

themauler said:

Clutch/driveline issue? Fuel delivery problem? Sounds like it could be anything. Pop the rear driveshaft off and front wheel drive it around the block see if it goes away?

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I would look at the clutch and pressure plate before anything else. A worn or sticky clutch plate would grab then slip then grab then slip under light load and low speed. My DD which is a 1994 Toyota pickup with a 5 speed a manual have been doing this for a couple of years. When it does this, I switch into the next higher gear and slip the clutch longer. This never happen when the pickup is loaded or taking off on a hill.

ftwelder said:

Mine XJ does something along these lines and figured it was due to the soft OME rear lift springs and the amount of exposed spline on the slip yoke. I have to push in the clutch to get it to stop rocking.

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A weak rear suspension would also do this but I would doubt this is the issue. Many on this forum complain about weak and sagging rear but never about wheel hop at take off.

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Ftwelder: Yes what you describe is precisely what happens to me. If I slip the clutch a little or shift into second it's smooth.

It's original clutch/pp everything. The previous owner did a complete fluid change about 40k ago (tcase, tranny, f&r diffs) so I might change rear differential fluid either way and get a look at it. I am leaning more towards leafs as the culprit as it did it today when I was driving on some washboard gravel roads.

I do get a soft clunk sometime when shifting gears from the rear end. Not violent but noticeable if the rpm's and not correctly matched to vehicle speeds.

slychicken37 said:

Ftwelder: Yes what you describe is precisely what happens to me. If I slip the clutch a little or shift into second it's smooth.

It's original clutch/pp everything. The previous owner did a complete fluid change about 40k ago (tcase, tranny, f&r diffs) so I might change rear differential fluid either way and get a look at it. I am leaning more towards leafs as the culprit as it did it today when I was driving on some washboard gravel roads.

I do get a soft clunk sometime when shifting gears from the rear end. Not violent but noticeable if the rpm's and not correctly matched to vehicle speeds.

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I don't think changing diff fluid is going to help. My jeep with an ax15 bucks some and I get clunks from the rear but I take that as just having a direct driveline. I never had that before when I still had an AW4. Ahh the comfort of an automatic.

You just have some slack in the driveline because when you feather the throttle going slow or your tires roll off of a speed bump the slack in the driveline goes away and tries to push the flywheel and then the engine resists it and you get a push pull feel. Mine has been doing that for a while now and I just roll with it. Can you rotate the rear driveshaft a little back and forth while the jeep is parked and on the ground? Mine twists like a quarter of a turn back and forward.

themauler said:

I don't think changing diff fluid is going to help. My jeep with an ax15 bucks some and I get clunks from the rear but I take that as just having a direct driveline. I never had that before when I still had an AW4. Ahh the comfort of an automatic.

You just have some slack in the driveline because when you feather the throttle going slow or your tires roll off of a speed bump the slack in the driveline goes away and tries to push the flywheel and then the engine resists it and you get a push pull feel. Mine has been doing that for a while now and I just roll with it. Can you rotate the rear driveshaft a little back and forth while the jeep is parked and on the ground? Mine twists like a quarter of a turn back and forward.

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I can rotate the driveshaft when the jeep is parked on the ground but not a 1/4 turn..... maybe an 1/8th turn maximum. Today I tried some of the above options described below:

1. I removed the driveshaft. The u-joints all felt fine with no play or slop anywhere. Needle bearings felt smooth but I did not take off the bearing caps and regrease.....they felt fine so I left them as is. Regreased the tcase output splines on the slip yoke even though it has nicely lubed already.

2. Road tested without the rear driveshaft on. Symptoms are about the same and almost worse. If I turn I get steering feedback with a rocking motion in the wheel. Not for sure what that is about. (I get it when I am in 4WD with or without the rear drive shaft)

3. Both the rear pinion and front tcase output shaft are solid. No play at either bearing or fluid leakage around the seal.

4. Reinstalled everything and re-road tested. Did the same as before.

I still have not rulled out carrier bearings, but maybe its just a jeep thing and I should just smile and drive it!

One thought was mounting a GoPro under the vehicle and recreating the symptoms so we/I could get a visual of whats going on. Could be driveline slack or rear leaf spring slack which would be evident in a video.

What condition are the leaf springs in? Mine does this if I load up the rear end, and the tires break loose. What I'm getting is wheel hop from my worn out springs twisting up.

I am not for sure on the rear leaf springs....how can you tell?

When the jeep sits the rear leafs are basically flat under compression with nothing in the back.....should they still be convex shaped when sitting without load?

Maybe one of the springs in the clutch disk broke and is causing slop. Not really worth taking apart unless the clutch is bad. With the transmission in 5th gear, and the back wheels off the ground, can you get the drive shaft to turn back and forth?

slychicken37 said:

I am not for sure on the rear leaf springs....how can you tell?

When the jeep sits the rear leafs are basically flat under compression with nothing in the back.....should they still be convex shaped when sitting without load?

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Mine are flat as well, and that's with an AAL in them! I think they should have a curve to them.

For those still following this thread I spent yesterday and took off the rear differential cover. I attached a few photos but everything seemed tight and good to go. Used a pry bar but could not create any movement in the carrier or pinion.

Still haven't filled out suspension.

Just for clarification on my part, how many connection points are there for the transmission, transfer case, and engine? I have already replaced the rubber transmission mount and drivers side motor mount. So is it just the three rubber mounts that keep everything front pitching under load?