Check your driveshaft center bearing. Mine was shot and I had similar symptoms. It was obviously bad upon inspection...the rubber was falling apart. Replaced, and no vibration since.

Good Luck

__________________
1984 Astral Grey 300TDT - The French Fry
1979 Yellow 300SD - The Pomme - SOLD
1985 Light Ivory 300DT - Fritz
1987 Anthracite Grey 300SDL - ?