Its hard to gauge degrees of "clunk" on a message board, but my 1982 300D with 325K also has a clunk to it that is almost certainly caused by looseness in the differential.

One symptom that is a little tricky getting used to is that when I put the car in Park, it may continue to roll for a few inches. That can be disturbing when parked in close quarters!

I've had the cover off my differential due to replacing the half-shaft axles. The insides looked to be in good shape. From what I could see, the "slop" or "looseness" appeared to be part of the design, and not from wear. There is a shaft in the center of the insides of the gears that holds one or two smaller gears in place. Well, that shaft is fitted into a slot, which allows it to move back and forth to some extent.

I don't understand the design or the reason for the slot, but the car is doing fine and I can stand a bit of a "clunk" under the circumstances you describe.

Maybe other W123 owners will chip in with their observations.

Ken300D