Quick check in - ujoint?

Vibration under driver's seat when accelerating only, and more than ~30mph and pulled front driveshaft with no change. u-joint in rear driveshaft going out or should I check something else too?

Check for a sagging transmission mount allowing the exhaust to touch the crossmember during accel

most likely a ujoint i would say, yes. especially since it's only under load. it's easy enough to pull the rear shaft and check it.

since the XJ is a unibody you can't always trust where you hear or "feel" a sound or vibe coming from, they travel through the whole body easily. you're on the right track though by pulling the front DS and checking that first. if the joints in the rear shaft look good give the rear pinion and the output on the tcase a good hard shake, there should be no play in either.

Digger87xj said:

Check for a sagging transmission mount allowing the exhaust to touch the crossmember during accel

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also a good thing to look for. this usually makes a clunk when popping your foot off the brake in reverse though because of the direction of torque.

The other night when I took the front shaft out I shook the shafts by hand, kicked the crossmembers, checked all the arms on the suspension, nothing seems loose or bad. I'm going to pull the rear shaft and see if the joints feel weird. I've had experiences in the past where even if they move fine, there was something off about one and replacing it fixed the issue. Maybe I'm just not good enough to tell the difference?

Add motor mounts to your list of things to check.

spinaldex said:

The other night when I took the front shaft out I shook the shafts by hand, kicked the crossmembers, checked all the arms on the suspension, nothing seems loose or bad. I'm going to pull the rear shaft and see if the joints feel weird.

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When you pull the rear shaft, don't just look at the U-joints. Put the transfer case in 4wd and drive the car to see if the noise is gone.

XJFrankenstein said:

Add motor mounts to your list of things to check.

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Great point. Thanks!

MoFo said:

When you pull the rear shaft, don't just look at the U-joints. Put the transfer case in 4wd and drive the car to see if the noise is gone.

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So, put the front shaft back in since I know it isn't that. Remove rear shaft. Drive it. Check.

just for anyone reading this thread later, since your sig lists your XJ as a 2000 that is fine advice. 95 and earlier 231 tcases will puke all the fluid out of the back if you remove the rear driveshaft without having a slip yoke eliminator, the later cases have a sealed extension housing.

When my U-joints started going out last year, it felt like I was driving on a rough road pretty much no matter what the quality of the road actually was. U-joints are cheap, and replacing them, while kinda've a chore is pretty damn easy.

Pull the driveline, put new joints in, see if it goes away. Even if that isn't the issue you won't have to worry about them for the near future.

Thing I found when going through the debugging process is I thought it was engine related (1990 renix XJ) and started there, after checking the usual "running rough" problems I thought motor mounts, but realized that was more of a PITA than the spiders. Did those, problem solved.

Unless you checked the rear driveshaft with the vehicle parked in neutral, torque loading probably masked any issues it might have.

GrimmJeeper said:

also a good thing to look for. this usually makes a clunk when popping your foot off the brake in reverse though because of the direction of torque.

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I had a strange sound/vibration on startup and whenever the Jeep was stopped in gear (not in park or neutral) that drove me crazy. It was pretty loud, especially at stop lights. I could have sworn it was from the transmission. I could feel the vibration through the floor and the shifter.

It turned out to be the exhaust -- the muffler, actually. I only found it because I pressed on the muffler in a particular way when someone started the jeep -- no sound or vibration. I cinched the muffler up with some safety wire and hose clamps. It looks pretty ghetto, but it has been fine since.

I'm not sure, but I think something's busted inside and my cinch job stabilized it somehow. (What I did didn't move the exhaust away from something it was vibrating against before.) It has been six months or more and it still sounds fine.

Like someone else said, these unibodies seem to carry sounds and vibrations in all types of strange ways. In my case, it was traveling from the muffler through the exhaust, to a mount somewhere (I guess...) and up through the driver's side floorboard/shifter. I hope your fix is as simple... Good luck.

The way I check my u-joints is to jam the shaft (however you can, pipe wrench, pry bar, big screwdriver, whatever) then pry against the opposite yoke with a pry bar stuck through the joke. I look real closely at the rubber grease seal around the bearing cap. Any movement of the "X" end inside of the cap usually means it is junk or soon to be junk.

I typically replace 3-4 front axle u-joints for every driveshaft u-joint I need to replace. I always check the front axle U-joints first.

Front right spindle and axle were replaced a month ago, so I guess the other side could be bad but I'm still thinking its the u-joints. I'm just going to buy 5 of them and replace them all in the front and rear .. why not. Sure its a decent chunk of change at that point but it sucks when they grenade and I'd rather do preventative. Anyone know of a place that has cheap ones? I've seen some at $15-20 online, that about the best I'll find?

As for everything else, I shook and kicked the tranny mount, exhaust and all the suspension components and they are all tight.

U-joints:
Front axle (95 and later XJ, 94 and earlier also but only if equipped with ABS or upgraded shafts) 5-760X Spicer
driveline (all 5 of them) 5-153X Spicer

You can get them reasonably cheap from northerndrivetrain.com. I'd recommend getting two 2-70-18x U-joint strap kits as well, you're technically supposed to replace the straps every time you take them off. I'll reuse if they are in good shape/I don't have any new ones or I'm doing a trail fix but otherwise they get new straps.

I like to run a 1/4-28 fine thread tap through the holes in the pinion yokes before bolting the new straps on, too. They are generally packed with dirt and rust and I used to nearly round the bolts off every time I pulled them out, I haven't had an issue since I started doing this every once in a while. Don't use anything except a 6 point socket on those bolts, either, especially if you're using an 8mm (they're actually 5/16, an 8mm is a little loose on the bolts.)

I'm not at home (so I can't see what I have) and wanted to order them. Should I get greasable ones with a fitting or not for the drivelines?

Nevermind, found a good thread here on the subject:

http://www.cherokeetalk.com/forum/f8/greasable-u-joints-vs-non-greasable-19395/