I have a similar situation with my '87 560SEL. The motor mounts are OK and the flex disks were just replaced to quiet a vibration at speed.
When I first tried to get an answer to the harsh shifting into reverse from a couple of shops, I ended up talking to the manager at the local Aamco. As soon as I finished describing the problem, he indicated that this is the one area of weakness in the transmissions of these cars - if you can call something that usually doesn't show up until 150-200K miles a weakness. Evidently the clutch pack that is involved in shifting into gear wears thin requiring extra travel by the shifting servo. This causes an extra long delay and, due to the faster speed that things are now turning due to the delay, a harsher engagement especially into reverse. His recommendation was simply to drive it as long as I can stand it and then have it rebuilt for around $2,200. He indicated that the is very little likelihood of it causing further damage and that the worst that will happen is that I'll loose reverse.
MBDoc - does this sound like the service bulletin you were referring to?
'87 560SEL 267K (177K on motor) Blue/Blue
'98 Buick LeSebre 60K (wife's car)
'56 Imperial Sedan 124K
'67 Dodge Monaco 130K (Sold)
'87 Chrysler 5th Ave 245K and going strong (sold)
'73 Plymouth Satillite 175K (sold)
'96 Chrysler LHS 80K (totaled)