This sounds very familiar. I have an 300SL with 113,000 miles on the clock. It seems as if you have gone through the same process of replacing steering and suspension parts exhaustively with no results (but lots of expense).
My car often feels 'lumpy' in that it moves around on the road quite a bit. Diff does clunk and the are other squeaks and creaks from the rear at low speed. The vibration can be felt through the seat - not through the steering. I have recently replaced the pinion seal at the rear as there were quantities of oil being sprayed over the underside but I thought this was just a damaged seal rather than major diff problems. Oh dear!