Drivetrain Vibrations

Ok, so I searched and some people had some similar issues but nothing that seemed to match exactly the issue that I'm having.
98 xj, 4.0 205K miles, auto AW4 tranny, NP231, 3.5" skyjacker add a leaf lift with quick disconnects and transfer case lowering kit. I have had this jeep for about 5 years now and this problem just started less than a week ago. The tires were rotated less than 1,000 miles ago.
Now the problem, I have drive train vibrations that start at 35 mph and smooth out and completely disappear at 55 mph. It is definitely drive train related as when you let off of the accelerator the vibrations stop. U-joints don't seem to have any play in them and there are no other issues. Any ideas on what is causing this? I am off work for 4 days starting tomorrow and I'm going to tackle this issue and hopefully resolve it as this is my DD. Any help greatly appreciated! Thanks!

did you check the wheel end u-joints? i have the same vibration. but its from the lift i put on. i haven't lowered the t-case yet.

check the driveshaft and make sure nothing is stuck to it. or missing

I haven't checked the wheel end joints yet, I will do that first thing tomorrow. Wouldn't that vibrate even with the tranny in neutral? This only vibrates under acceleration.

Samson04 said:

I haven't checked the wheel end joints yet, I will do that first thing tomorrow. Wouldn't that vibrate even with the tranny in neutral? This only vibrates under acceleration.

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Transmission mount busted?

You know, I haven't checked it in a while. I put Brown Dog motor mounts on back in the summer and it was ok then but haven't looked at it since. Would that cause the vibes?

Samson04 said:

You know, I haven't checked it in a while. I put Brown Dog motor mounts on back in the summer and it was ok then but haven't looked at it since. Would that cause the vibes?

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I can't say for sure, but it can stress the drive train alignment, and stress of u-joints....and let the back end of the transmission jump around under acceleration. I found out mine had no bolts holding it in place when I replaced the front drive shaft joint. It only made a noise when under hard slow acceleration turning a corner, like a take off going around the corner. I think I heard and caught it early on mine.

I will definitely check it tomorrow. That's easy enough to replace. Would I be money ahead to just replace the drive shaft u-joints while I'm at it or should I wait until they fail?

Do you have a posi rear? Two of our fleet had D35 posi rears, one works fine, and the other had a really odd wiggle/squirm/vibe that would occasionally happen. The clutches were shot, and the alternate grabbing/slipping under gentle to brisk (not floored) acceleration was really odd. The pinion also had some slop in the bearing, so we replaced it with an 8.25, and the oddness subsided. Still chasing other gremlins, though.

Through the years I've had various driveline irregularities that were due to tires being out of round, out of balance, or lug nuts being too tight or unevenly torqued. About a month ago, I got new BFGs at Sam's Club. Free mount and balance, so a $60 savings. One of the Coopers was slightly out-of-round, so it lived on the back for the last two years as I couldn't rotate to the front. Anyway, driving home with the new tires, and it was like WOW, something's out of whack! Got home, put it up in the air, and on each wheel, one nut was TIGHT, and some of the others not so much. Oh, and they ran them on with a gun. I re-torqued them, in four steps, to the 80 ft.lbs. that they should be, and all is well. I looked at the sales receipt, and noticed that they claimed to have torqued them to 100 lbs. I've had this argument with various wheel jockeys through the years, and just re-do them when I get home. You mention having had the tires rotated recently. Check the torque, if only for peace of mind.

Now that you mention it, when this vibration first started was a little less than 2 weeks after the tires were rotated. I checked all the tires first thing and the passenger side front, 3 lugs were extremely loose and the other 2 weren't tight. I torqued them all down and drove it and thought the vibe seemed slightly better but when it persisted thought it must have all been in my head. What are the chances that it damaged the tire?

heyhar said:

Do you have a posi rear? Two of our fleet had D35 posi rears, one works fine, and the other had a really odd wiggle/squirm/vibe that would occasionally happen. The clutches were shot, and the alternate grabbing/slipping under gentle to brisk (not floored) acceleration was really odd. The pinion also had some slop in the bearing, so we replaced it with an 8.25, and the oddness subsided. Still chasing other gremlins, though.

Through the years I've had various driveline irregularities that were due to tires being out of round, out of balance, or lug nuts being too tight or unevenly torqued. About a month ago, I got new BFGs at Sam's Club. Free mount and balance, so a $60 savings. One of the Coopers was slightly out-of-round, so it lived on the back for the last two years as I couldn't rotate to the front. Anyway, driving home with the new tires, and it was like WOW, something's out of whack! Got home, put it up in the air, and on each wheel, one nut was TIGHT, and some of the others not so much. Oh, and they ran them on with a gun. I re-torqued them, in four steps, to the 80 ft.lbs. that they should be, and all is well. I looked at the sales receipt, and noticed that they claimed to have torqued them to 100 lbs. I've had this argument with various wheel jockeys through the years, and just re-do them when I get home. You mention having had the tires rotated recently. Check the torque, if only for peace of mind.

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I had the loose lug nuts problem on my 87 Christmas night this week! IIRC it mostly showed up during low speed acceleration and turning a corner.

I sometimes wonder if the lug nuts can be tight, and then once the wheel is loaded and driven, the wheel shifts out of a bind, and suddenly the lug nuts are loose. I have that problem several times the last 40 years.

Probably didn't damage the tire, but what wheels are you running? We've got four XJs, and six sets of tires/wheels. All wheels are genuine Jeep alloys. I've had various aftermarket wheels through the years, but find that 'real' wheels are the best. And, we're choosy about which brands of tires we'll use. I mentioned Coopers, my son bought them, and they ended up on my '96 Country. I even had them 'Hunter Road Force' balanced, and still, the one was banished to the rear, as it just wasn't right. Right at about 62 MPH, there was a weird vibe, and was mostly gone by 65-70. And, with only about 30k miles, it was time to replace them for the winter.

It's been my experience with four XJs and a ZJ over the last fifteen years, that they are sensitive to balance, roundness, and proper torque, more so that a vehicle with a more 'modern' suspension. Also, I'm mindful to a tire with a sidewall stiffness rating about what an OEM tire would have. Cherokees are rather light vehicles, and too stiff a sidewall, especially for a strictly street driven vehicle, like ours are, is not the recipe for a satisfactory ride.

I like to torque the wheels in four steps, 30, 45, 60, and finally 75-80 lbs.ft. This will allow the wheel to seat evenly on the hub, hopefully giving the smoothest ride. When I see a 'tech' jamming the nuts on with a gun, I know the torque wrench is coming out as soon as I'm out of sight. Each of our five cars has its own torque wrench, and I keep one in the garage.

Samson04 said:

You know, I haven't checked it in a while. I put Brown Dog motor mounts on back in the summer and it was ok then but haven't looked at it since. Would that cause the vibes?

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It has been suggested by others to be a good idea to swap tranny mount when replacing motor mounts. I think the theory is all three wear and settle together then new motor mounts pull on the worn tranny mount.

I think brown dog suggests loosening, inspecting and retightening the tranny mount at least during motor mount install.

Mine looked OK when inspected, and not terrible when removed, but I have heard others say theirs looked OK but pulled apart like a peanut butter sandwich when removed.

heyhar said:

I like to torque the wheels in four steps, 30, 45, 60, and finally 75-80 lbs.ft. This will allow the wheel to seat evenly on the hub, hopefully giving the smoothest ride. When I see a 'tech' jamming the nuts on with a gun, I know the torque wrench is coming out as soon as I'm out of sight.

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I was the same way about battery clamps after a shop worked on any of my rigs. They always forget to tighten them.

I like the multi torque idea. I usually try to get them seated and sort of snug before I let the vehicle back down!!!

Samson04 said:

You know, I haven't checked it in a while. I put Brown Dog motor mounts on back in the summer and it was ok then but haven't looked at it since. Would that cause the vibes?

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Yes.