When you put your gear selector into drive or reverse and you hear or feel a big clunk, it's probably work out rubber flex discs on your driveshaft, or possible a worn out center support bearing on the driveshaft.
It could also be broken differential mounts, but my money is on worn out flex discs.
You need to get this looked at right away. If it's a worn out flex disc, and it fails when your on the road, you could be looking at some expensive damage.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".