I've got the same thing on my '82 300D. I have the differential open now for an upcoming replacement of the two axle half-shafts it uses. There is some sort of floating gear in the differential that is on a shaft that moves back in forth within a slot. I'm sorry that this is not all that descriptive in words. I might be able to post a picture.
Anyway, my differential does the same thing - the clunk is probably that gear reaching the end of its "loose" travel. I'm going to spend some time in the manual to attempt to figure out what this design does.