Contrary to the popular opinion in this thread, I will bet you money that it's actually a sticking slip yoke, which is VERY, VERY common on the XJ.

Easy fix, too.

Get under the Jeep, unclamp the rubber boot at the front of the rear drive shaft, slide it out of the way, and mash a bunch of grease into the splines of the drive shaft. Re-clamp the boot, and go for a drive. Within a few stop/go's, it'll stop clunking.

I'll bet you anything that this is the problem.

 

It felt a lot better. Just drove it to work this morning, mostly highway. I will be checking my rear diff. tonight.

 

I'll try that tonight, what type of grease to you suggest?

 

Any axle grease should work fine. Ideally it would be good to clean the old crap off first (or what's left of it).

 

got it, very easy, about to take it out for a drive. thanks bud

 

I have a similar clunk coming from underneath the front seats (could only hear it after a far bigger clunck from the rear axle was resolved after replacing engine- and transmount).

So, did greasing the slip yoke solve it?