I had a similar problem with a 350SE I owned. I took to my trusty MB mechanic (notice I said MB mechanic and not Trasmission shop). Turns out the transmission could be adjusted (reverse at least) apparently on that transmission there was some sort of pressure nut that could be tightened. Alleviated the problem whereby the car seemed to be straining to move backwards.
Sounds like yours might be a bit more serious. Stuck vavle, binding valve, dead pump etc.
You might want to have it diagnosed by your local MB dealer and then decide what to do from there.
Used MB trannies really aren't that expensive (depending upon model).