Thrumming from rear end?

Jeep info:
1990 XJ, 4.0L, AX-15, D35 w/lsd, 180K.
3" Tomken lift with stock driveline.

My XJ has recently developed a thrumming/throbbing noise from the rear end. It's a "whump-whump-whump" that is periodic, and which starts once the XJ is above 30mph or so. There's no vibration that I can detect.
Also, the noise only occurs when I'm on the gas/under load (disappears when I back off the gas).

Lift has been on for the past 5-6 years with no change, so I don't think its driveline angle or anything like that. I also don't suspect wheel balance, since that was done 2-3 months ago, and the noise isn't always happening (again, only when on the gas).

The only things I can think of are ujoint issues, or possibly the rear diff going bad. Any ideas or suggestions of what is wrong or what to look for are greatly appreciated.....

I would start with inspecting/testing the u-joints, check the torque on the lugs, put it up on jacks and rotate the tires checking the tread face for any irregularities.

Thanks Joe. I checked out the ujoints (didn't find any play) and lubed them last weekend, but still had the problem. The tires were rotated/rebalanced about a month ago, so I'm thinking the tires are okay; I'll recheck the torque on the lugs. Could warped drums do this?

U-joints or a pinion bearing. "Looks OK" and actually being OK are not always the same.

What's the best way to test the ujoints and pinion bearing?

U-joints are a routine maintenance item. If they are broken, worn out, or making noises, you simply replace them. Low cost part, simple driveway job.

Often a bad pinion bearing will be leaking gear oil at the seal, will be loose, or the drive line still makes that noise after you have replaced the u-joint.

Tim_MN said:

U-joints are a routine maintenance item. If they are broken, worn out, or making noises, you simply replace them. Low cost part, simple driveway job.

Click to expand...

I'll vouch for simple. Took me 40 minutes to do both ends and that included resurfacing the yokes. I dreaded doing U-Joints but it turned out being so simple that it was actually fun to do.

...and my annoying vibes, clunks and thuds disappeared. Nice driving now!!

We're chasing the same thing in our '96 Classic, with the Up Country 1" lift. Same d35 LSD. Used to quiver under light acceleration, extra tube of posi goo has helped. Now it has a light vibe, comes in at about 60-65 MPH, not bad, but sort of like the OP has described. I'm thinking pinion bearing, but who knows? Then just today, my son said he's feeling it in his '95, also d35 LSD...

Anyone got any 8.25s on sale Buy One-Get One?

I was able to change out the front and rear ujpoints this past weekend. It didn't make any difference; the thrumming noise is still there. While the driveshaft was off, I did notice freeplay in the pinion yoke. It was rotational freeplay;the yoke seems to have 10-20 degrees freeplay. I retorqued the pinion nut, but the freeplay was still there. The yoke didn't seem to have any in/out or side/side freeplay, and no leaks either.
Does this mean the diff is bad? Better to swap it out than fix?
Thanks

since your D35 has the LSD option, i would pop open the diff cover. mine was making noise so i took off the cover and found chunks of my LSD sitting in the bottom of the diff. and if that looks good as well. i would suspect the pinion bearing, like stated above. it would be a good time to swap in a 29-spline 8.25. thats what i did when my D35 took a poop

If it is the lsd, can I just pull it out?