Any benz with good OEM shocks shouldn't "bump" like that. I would suspect that all four shocks are going, if not gone. Do you also have porposing over dips, or excess sway on curves?
The thump cannot be the suspension bottoming out unless you have hit a really big bump -- there is an amazing amount of travel. I think that what you hear is the shock only working for part of the suspension travel -- free movement up to where it starts to work, then it thumps. Also, check the rubber mounts on the top mount -- they can go bad, and if loose, will thump.
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!