Vibration above 45, goes away off throttle

Hi,
My XJ has developed a high speed vibration, that wasn't there before. At any speed above about 45, it will vibrate pretty strongly, through the chassis (not the steering wheel). Once I let off of the gas, it goes away 95%, the second I step back on the gas, it comes back.

U joints are fine, front and rear, I've checked them twice. I've tried rotating the tires to rule that out, no difference. Fluid levels are fine in the trans, xfer case, and diffs.

XJ is a 91 Limited with 2.5" custom leaf pack rear, spacer front, Bilstein 5125s, boxed control arms, 242, and 241k miles. Car drove fine with the lift until this developed.

I noticed that the motor mounts look like they are sagging a bit (I can fit my finger between the rubber and the top on the passenger side). Would that have anything to do with this?

Any ideas? Thanks!

rear pinion angle.. do you have a sye or T-case drop kit?

I would check your angles

Might check your tranny mount, but it really sounds like a rear U-joint. I had one go bad that was giving me very similar characterisitics (hit a certain speed got vibes, got off the gas they went away). I checked it, took it to a Midas and they checked it, got it home and got ready to pull the drive shaft and third time was the charm. My problem was that there was still torque in the shaft because the Jeep was on a bit of an angle, might try jacking the rear axle off the gorund and checking it if you haven't tried it that way

ADVNTURR said:

Might check your tranny mount, but it really sounds like a rear U-joint. I had one go bad that was giving me very similar characterisitics (hit a certain speed got vibes, got off the gas they went away). I checked it, took it to a Midas and they checked it, got it home and got ready to pull the drive shaft and third time was the charm. My problem was that there was still torque in the shaft because the Jeep was on a bit of an angle, might try jacking the rear axle off the gorund and checking it if you haven't tried it that way

Click to expand...

what he said!

No SYE or tcase drop, as it didn't vibrate. I changed one Ujoint about 2 years ago. I'll just pulloff the driveshaft and check it, it is easy enough.

Thanks!

Put the ebrake on, chalk the tires and then put the tranny in neutral. this will take the load off the driveline so you can check the u-joint. Another possible issue could be your driveline threw a weight and is now out of balance.

those motor mounts are probably contributing, go with some M.O.R.E bomp broofs, or i think Rawbrown has some nice rubber ones

It was the U joints...I went out and put on the e-brake and put it in neutral to uload the driveshaft. Front one (on rear shaft) had a little tiny bit of play, rear was done. New joints are in already, I love how easy it is to get the DS out of an XJ.

Thanks for your suggestions! New motor mounts will go in when the stroker does, as I have 75% of the parts already.

got the exact problem going on right now.

checked the ujoint out back last night, it looked ok... though i was in drive and probably loaded. will try again in neutral.

also, reading this post and a few others. i notice the motor/tranny mount is mentioned a few times. i have a feeling they may be contributing. how does one diagnose these?