Rear driveline still vibrating/grinding. I have checked just about everything.

My Jeep has been grinding/vibrating for like the past month and a half now, it still drives "ok" and i have put prolly 2000 miles on it like this but it is driving me crazy. I have tried just about everything I can think of and I still cant find the problem. For the record I am running a stock 2001 slip yoke/driveshaft on about 3.5 inches of lift, NP231 t-case and an Aussie locked 8.25.


Problem:
If you are on the gas any significant amount over ~55 it vibrates really bad, like it feels like you have a massaging seat. If you are coasting at any speed over ~30 it doesn't vibrate but it makes a somewhat loud clinking/grinding sound. If anyone has heard a really bad driveshaft slip-joint it sounds exactly like that.

This started right after I did my gears, however I'm pretty sure it isn't directly related, there is no sign of abnormal wear on the gears or my aussie locker and I have changed the fluid twice since the break in period and there is no excessive metal shavings or anything out of the ordinary in there.

At the same time as the gears however I also messed around with the leaf pack giving it an additional inch perhaps of lift in the rear and removed the shims that were in there. The removal of the shims brought the pinion down from pointing well over the angle it needed to be (pointing way too high since lift shackle install) to pointing directly at the T-case. At first I thought it was the incorrect pinion angle causing this so I threw a 2 degree shim in there. It made the angle much better, it is still maybe ~2 degrees too shallow but it made absolutely zero difference with regard to the problem.

I changed the t-case fluid this morning, no metal shavings or anything out of the ordinary. The case also shifts fine and behaves normally in 4wd. The rear output yoke has absolutely zero play, the front has maybe a 16th of a turn which is supposedly normal, the rear axle yoke has about this much as well but it is normal with a locker.

The front driveshaft was totally rebuilt about 6 monthes ago, I even did the centering ball. The front slip joint doesn't have any play.

I jacked the rear end up and ran it at about 55, it sounds like the sound is coming from the t-case or the driveshaft, pretty sure it isnt higher or lower on the driveline. While driving it sounds and feels like it is coming from right under the passenger seat.

So Basically

Stuff i've checked:

U-joint's
Axle gears
Stretched t-case chain
nothing obviously wrong with the t-case
anything dumb like loose u-bolts
Pinion angle (pretty sure)
Front driveshaft


Anyone have any more ideas short of throwing parts at it or taking the t-case completely apart to check for some funky ass internal problem? I cant think of anything more to even look at. Gonna talk to my driveline teacher at school tomorrow too and see if he has any input.

Well youv'e got 2 areas that cause problems.97+ don't like being lifted without installing a SYE and youv'e got a 8.25 which can be really problematic when doing gears(you didn't say what ratio).

"My Jeep has been grinding/vibrating for like the past month and a half now, it still drives "ok" and i have put prolly 2000 miles on it like this but it is driving me crazy. I have tried just about everything I can think of and I still cant find the problem. For the record I am running a stock 2001 slip yoke/driveshaft on about 3.5 inches of lift, NP231 t-case and an Aussie locked 8.25."

You have a post 1997 XJ with a lift over 1.5 inches and no SYE. Simple as that. You need to add an SYE to eliminate the driveline vibes before you destroy your u-joints and t-case. You can try a TC drop and see if that helps. Also can try shimming your leafs with 4-6* shims.

add another vote to an SYE

another vote to SYE

I would have the driveshaft balance checked. I also vote for SYE.

try packing some heavy grease into the slip yoke, if that doesn't work, SYE.

also... I think you may be confused, or using the wrong terminology and confusing me. You have no SYE, but your rear axle is only a few degrees from pointing at the transfer case? That's bad. You want your pinion to be 1-2 degrees below parallel with the transfer case output shaft if no SYE or 1-2 degrees below parallel with the rear driveshaft if you have an SYE.

My '99 did the same thing with the grinding after you let off the gas before I did a SYE. The vibrations and grinding never came back after the slip yoke eliminator and new driveshaft.

CharlesS said:

To clarify the need for a Slip Yoke Eliminator...
In around 1994/95 the transfercase output shaft support bearing was relocated slightly for an output shaft seal redesign.
The relocation of the bearing caused a change in the support of the yoke.
When you install a lift even if its only 2", the drive shaft slider moves outward slightly which can change the angle of the U joint for the yoke and drive shaft causing vibrations.
On some vehicles the U joint actually binds slightly causing increased vibrations.
An interim fix is to replace the yoke on the drive shaft with a yoke from a YJ, but the best fix for this problem is cutting down the output shaft slightly and installing a fixed yoke on the output shaft, modifying the drive shaft for a double cardan joint to bolt to the fixed yoke and shimming the rear axle approximately 4*.
My 1998 4.0L with NP231, AX15 and C8.25 had vibrations from when it was new in Oct 1997. The dealership came up will all the usual excuses and was never able to do anything....
The ONLY fix was having a SYE installed by MIT in April 1998.

Click to expand...

yep, 95 (possibly mid year change but I don't think so) was the last year of the old design, 96 started the new design. My 96 has the new one... and some vibrations.