I would bet on a bad flex disk, trannsmission mount and/or a bad carrier bearing support. Either will allow the driveshaft to vibrate at resonance speed (about 40 mph in your case).

I'd replace the front flex disk as a matter of course, they only last about 60,000 miles in these cars because the "softening" molded in fails. Gives a harder shift than you should be getting, too.

The u-joint in the driveshaft is not replaceable, should give you an indication of how often it needs to be done.

Rear axles can go bad, too, but that vibration is distinctly from the rear.

Check the differential mounts in the rear subframe as well by jacking on the diff -- if the bolts on the rear are'nt centered well and move from the lower part of the mount a lot when you jack on the diff, they are shot. Big PITA to change, but worth it.

If the problem is much worse under hard accleration, it's probably a bad tranny mount.

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!