97 XJ needs advice.

OK, here's the problem in a nutshell. I have a 97 XJ with a Rubicon 3" spring over add-a-leaf suspension lift with 31/10.50 AT. The truck runs awesome, except for the loud rumble coming out of the back end. It really sounds the loudest at about 3000 rpm's or on the freeway at about 65 mph. But when you let off the gas it quiets down a ton. Some friends have described it as a vibration, I think it's more of a rumble it almost sounds like a metallic sound.
In my opinion the spring packs have also sagged quite a bit. I can't find any photos to confirm. That's more of an opinion. I replaced the u-joints on the rear drive shaft a couple weeks ago. Last weekend we decided to shim (2 1/2 degree shims) the rear diff to see that did anything. Neither did a thing.
So here I am back at square one looking for suggestions. My feeling is that it should not be this loud.
Does anyone have any suggestions or comments???

dID YOU REPLACE THE DRIVE-SHAFT u-JOINT AS A HUNCH OR BECAUSE THEY WERE WORN OUT? (oops - CAPLOCK).

It sounds like you have some pretty bad vibrations. Did you install a transfer case drop with the lift? If not that will be your cheapest, easiest fix. The best fix would be to get a slip yoke eliminator kit and a high angle drive line.

Did you put the thin end of the shim facing towards the front of the truck?

Also, after you initially lifted the rig, did you change the ujoints?

Basalt is right, a bad ujoint will often rumble like that, especially if it was run in long before you lifted or shimmed the truck...essentially the joints would be used to running at the old angle...now you're running at a slightly different angle...

Chock the wheels of the truck, put the ebrake on and the tranny in neutral. Go under and wiggle the shaft side to side...any play? Now hold the diff pinion in one hand and the shaft in the other...will it twist? Unless the rig is in neutral it will be very hard to determine if the joints are bad.

3.5 of lift is a bit much for many XJ's with no tcase drop, shims or SYE, so I'd suspect it's the Joints.

I agree with the above responses.

But it could be you are experiencing the notorious chysler 8.25 diff growl...

Do you have the D35 or the 8.25"?Yes the 8.25's are known to be noisy, but so is the 97+ transfer case/driveline.Have you actually "measured" the angles?You can try the T-case drop also,after that youll probably need an SYE kit.Even the Hack-n-tap kit didnt work for me and had to Re-invest in a HD Sye!

When mine was doing that, my pinion bearing was dying. I have a '97 that was at 3" with a 8.25" rear. If that is the problem, fix the driveline angles before long or it will happen again.
mattk

my 8.25 is quiet :listenmus

It sounds like driveline vibration, but could also be pinion bearings. First thing it to check and fix the driveline angle, and check for a bad u-joint. You can wiggle the rear of the driveline to see if the u-joint has play in it, but a bad u-joint will usually make a clicking sound when you're not going too fast.

If you don't have an SYE (slip yoke eliminator), the u-joint angles on the front and rear of the driveline (t-case and diff) need to be less than 3* difference. You'll need an angle finder to check it, most hardware stores have magnetic angle finders (around $5). If the angles are off more than 3*, you'll need to rotate the rear pinion by a shim under the leaf springs, on the axle spring pad. If your lift came with one, you may need one with more or less degrees, or you may need to turn yours around. Getting the angles right can be trial and error, but it's something that needs to be done. The vibration caused by bad u-joint angles can wear out both the u-joint and the pinion bearings in the rear differential. Unfortunately, usually the pinion bearings go first.

Merry Christmas,