In general Bad Flex disc will definately be a major factor. Bad engine /differential mount also are found to be the cause for hard 'D' and 'R' engagement.
I had same problem with my 87 420SEL, I have resolved the hard "R" issue by replacing engine and differential mounts but still have the hard "D" shift from "P". Still working on it, I have changed quite a few parts to eliminate the problem, eg engine mount, differntial mount, subframe bussings etc.

What is your rpm reading at idle? mine is 700--800 at idle.

Keep us posted.
Thanks

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87 420 SEL
Looking for a nice mid 80's MB diesel.
87 BMW 528e (Sold)
99 TOYOTA 4Runner
1989 Honda Accord (Sold)
1988 Honda Accord (Total)