Vibration under load

[myraus]

Hi. On my 1996 XJ, I'm getting a vibration under load, like when I accelerate. Usually it starts about 40 k but it isn't that noticeable at highway speeds.

Mine is a 4.0 AW4 NP242. I just replaced the trans mount and rear u-joints. The previous owner put in new rear spring packs from Standens last year which raised the rear approx. 2 inches from original ride height so now I'm gonna get 2" spacers for the front to even the rig out. Would such a lift cause the vibration?

[Dingleberry]

The lift itself isn't the cause of the vibration, but it can alter the angles of driveshafts/suspension components causing them to vibrate due to worn u-joints, bushings, etc.

Have you removed the front driveshaft to check its U-joints as well?

[myraus]

I'm gonna pull off the front driveshaft tomorrow and test it without it. Can the rear driveshaft be taken off and drive the XJ in 4wd {I have a 242} with just the front driveshaft on? I replaced the u-joints but the vibration is still there. Gonna go to pick-n-pull to find another driveshaft.

Also, is it hard to change the motor mounts? The other problem is that when I drive it and apply power the brunt of the engine vibrations come thru the chassis. When I let off the throttle it smooths out.

[bRIBEGuy]

Vibrations on lifted XJ's are VERY common, and can certainly occur with only 2" of lift.  Check your rear pinion angle, you might need to shim it a bit, or look at a transfer case drop or slip-yoke eliminator.

[dubbleJs]

Im pretty sure on the '96 242 you will leak all of your tcase fluid out if you drive with no rear driveshaft.

Have you checked the wheel balance? I would say it is either that or a driveshaft angle or ujoint issue.

[myraus]

Replaced the motor mounts this afternoon, which made a big difference. The vibration is still there and I think I've figured out what is causing it.

The transmission!

It's an AW4 and in 1st it feels fine, but in 2nd, 3rd, and somewhat in 4th it vibrates when I put some throttle into it. And diminishes when I let off.

Question is: how hard are they to R&R? Or should I just look for a used one and keep my fingers crossed that it doesn't do the same thing?

[bRIBEGuy]

Doesn't sound like a transmission to me, it still seems like your rear pinion angle is likely off.  My XJ had similar vibes after it's first 2" of lift, and it went away forever as soon as I put in a SYE.  Check your pinion angle, or splurge the $20 to do a t-case drop.  My 2 cents.

[Dingleberry]

How about checking the transmission mount as well, since you already looked at the motor mounts? The rubber in the tranny mount (cross-member) can wear out just as fast as the motor mounts.
Seems like the next logical thing to take a look at, if you ask me.