I had the differential mount replace a couple of years ago, but still a rather nasty sounding clunk in the rear. This year, Herr Gottfried replaced the upper and lower rubber spring mounting bushings (all available) and then discovered that the protective cover for the fuel pump was loose and also interfering with the suspension movement, Voila! The noise is gone. He returned the old suspension bushings (actually rather like large washers) and they appeared to be fine, so it might have the been the fuel pump cover all along. This is something quite easy to check.
But I also agree, that the rear diff mount, if collapsed can be a problem, fortunately that is a pretty easy fix.
Jonny B
1967 250SL Auto