I've been chasing the same problem for several months on my 82 240D (114K). I've done the following:
New motor mounts, Drive shaft mount, new shocks, rotated and balanced tires faithfully, Topped of differential, checked for bad boots or other signs grease, and everything seems to be tight. Also checked for broken tire belts, and replaced one back tire that had started cupping on the front. Allignment seems very good on front and I do not detect any unusual wear on the back tires. I still get this "bottom wobble" from 30-40 mph. out of the rear end. I'am running Goodyear Regattas with around 24K on them. I'am hoping we can both learn something on this one. Thanks a bunch!