The only other possiblities with this rear end are:
Loose junk in the trunk
Loose upper shock mount (since I assume you checked the lower one)
Split sway bar mount bushing that looks fine but allows the swaybar to thump
Loose or split differential mount, top (four bolts) or rear
bad or missing control arm bushing -- this usually makess the car "throttle steer" excessively, too -- pulls one way under trailing throttle, the other under acceleration (another thing wrong with the 220D). Very rare, but possible.
A broken parking brake shoe could do this -- I assume you don't have a wheel cover, so it couldn't be something loose in there, either.
And check for loose brake calipers or pads -- I'd be suprised if this were the case, but check anyway. If the anti-rattle spring is broken, it is possible for the pads to get loose.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!