I have an 1986 MB 560sl with 120.000 miles. I also had a big clunk when I put in R gear and slightly lower clunk when i put in D gear. The problem was in transsmission end I dont know exatly but the mechanic changed some rubber sealing and gaskets within transsmision, also the mounting and the clunk whent away. I had many mechanics looked for this problem and each of them had different opinion saying it is differential, driveshaft mountings...The only confusing thisng is that the transsmision sends a vibration to the driveshaft and the clunk can be heard at differential witch makes conclusion something is wrong with differential....This was not the case with mine.The problem was in transmission.


I hope it this helps.

Balsa