2001 Grand has "thump" in rear end

My 2001 Grand Cherokee developed a "thumping" sound in the rear area at low speed on bumpy roads. Started around 40,000 mi...now at 60,000 mi. Getting worse. At first I thought it was bad shocks and replaced them to no avail at 45,000 mi. I have looked at everything I can think of that might be a problem and cannot find any reason for the noise. Spare tire is tight, no loose items in spare tire well, suspension looks OK (no visual evidence of anything moving around), brake pads are secure, exaust is tight, ujoints OK, sway bar is tight, etc. I am begining to think that something in the rear suspension is loose or sloppy enough to allow a small amount of movement and cause a thumping sound as it moves around at slow speed. However, I cannot see any evidence of that. I have rebuilt Jeeps from the ground up and generally I can track down problems but this has me baffled. Has anyone had any problems with the rubber inserts in the rear suspension that might allow some slop in the joints? I suspect that is where the problem is but I don't want to tear it apart unless I have to. Paul D