I'm since sold the car buy the clunking came from rear strut upper mount. They unscrew and can be replaced. About $90 at the dealer.

Now I drive 94 S500 and there is clunking from right rear. In this car clunking comes from lower strut mount where it connects to control arm. Those mounts are part of strut. Strut has to be replaced. Struts are expensive and i would recommend replacing accumulators or convert to non slf level regular springs and shocks.
I'm converting rear self level suspension to non self leveling one. My strut is shot and accumulators are shot too. Conversion is cheaper and easier to maintain, just replace shocks when they fail. Spring have to be replaced also as self level suspension springs are weaker than non self level.

I hope this info helps someone.

__________________
1999 ML430 Silver 135k ml
Smoked LED taillights, Alpine in-dash DVD player, 12" sub, Pioneer components, 2 amps, 4200k HIDs, ML55 headlights, 20" rims, mirror LED blinkers

SOLD. 1996 E320 145k ml. Burgundy

SOLD but miss it everyday 1992 500sel Black
Chime buzzer mod,Alpine CD/MP3 player with stock amp,Euro headlights,S600 grill,20" rims, BEST CAR EVER!

SOLD. 1990 300E White
187k ml,smoked tails,Euro headlights,17rims,Intake,12" sub in first aid kit,alarm/autostart,buzzer mod