Wendell, you know.. I think i have possibly the same symptom on my 92 300e... Mine does the following:
In the mornings when i want to say.. reverse the car out of the driveway, i place the car into "reverse". i then go to accelerate the car a little as i reverse. i hear a single, pronounced "Clunk" out of the rear after a few revolutions of the wheels. If i happen to have my foot on the brakes when this noise happens, i can feel a definate pulsation in the brake pedal (!). After i reverse out, i will put the car into "Drive". As soon as i press the accel. pedal, i hear another single clunk emit from the rear. This behavior is baffling me, - i had the front flex disc replaced a few weeks ago, and it didnt change a thing. I may also mention that i do seem to have a vibration from approx 60mph+, if its of any relevance (Might be another issue, don't know.. i hope it's related!)
Is this anything at all like what you're experiencing??
92 300e 2.6