Dave, I went through a big learning experience about a month ago on my '92 SEL and the rear suspension, as I also had a "thud" in the rear when going over even tiny bumps in the road. Check out this thread:
http://www.peachparts.com/shopforum/showthread.php?threadid=85537
My guess is that you'll have to replace one or both rear upper shock bushings ($40 each) and both accumulators ($100 each) plus labor unless you DIY. If you don't take care of this asap you will be looking at a MUCH higher repair bill!
Good luck!