A bad flex disk will also cause a thump on downshifts, and a bad center bearing rubber mount will act almost like a bad tranny mount (which you should also check) -- vibration, almost hammering, under acceleration only.

I'd bet your engine mounts are shot too -- take a look at the driver's side (since it's easier to see) -- if you cannot clearly see the tapered portion at the bottom, or the lip of the rubber part at the bottom of the hemispherical metal portion is actually sitting on the frame, or you can see evidence of oily leakage, they are shot. The passenger's side will be worse, but you cannot see it, it's under the turbo.

My guess would be bad engine mounts definitely, and probably a bad flex disk too.

Peter

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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!