I have an 1982 300 D. I am having a similar problem. Slowing down from crusing speed at appox. 20 mph I have a "rattle" that appears to come from the differential. Also on acceleration if I hold at about 30 mph I have the same sensation. As I accelerate the rattle or vibration tends to fade away.

It's driving me nuts ! I have had it in the shop three times and nothing can be found. Is this similar to you situation? It may me an answer to quite my ride.

R. Tucker
1982 300 D 91,000