MB's don't have a live axle in the rear, so the driveshaft does not go "up and down" like an old Chev.
Your MB has a flex plate that is a rubber doughnut that absorbs the driveline shock.
Odds are it's worn out...
...replacement is usually a couple hundred bucks by most mechanics, and it's not that complicated for the DIY.
1998 C230 "Black Betty"