Is the vibration at wheel speed or drive shaft speed. Drive shaft speed will be same as engine speed in top gear. Feel the rubber engine mounts for splits or softening. Check along the exhaust system for possible body contact. How long since the tappets have been set. Tight tappets, not letting the valve close completely will cause a weird vibration. Coast down a hill in neutral, foot of the throttle. The engine is turning slow but the drive shaft is going fast. What do you feel then.
Let us know what you find.

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1984 300SD turbo 126
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