If the u=joint is bad, get a used driveshaft, it's cheaper. The u-joint can only be replaced by someone able to strake it in place AND able to balance of the shaft. Typical driveline place will look at it and go "huh? What's that?", and won't buy the necessary adapters ($1500 for the set) to get the shaft on the balancing machine.

Check both flex disks and centering sleeves and the center bearing and rubber mount before messing about with the u-joint -- there is very little flex in the driveline, so the u-joint would have to be VERY shot to make it vibrate.

If you can move the shaft more than just very slightly with a lot of force in the rubber bearing mount, you need a new mount. An eigth inch by pulling on the driveline is WAY too much!

Also, a dying rear axle half will cause the same vibration as a bad u-joint. The difference is that the u-joint will cause the floorpan to vibrate at the bearing mount (beside the front seat) -- if you put your hand on the driveline tunnel and can feel the vibration most strongly at the rear half of the front seat, its the bearing mount, flex disk/centering sleeve, or j-joint. If the vibration is more to the rear of the car, it's probably an axle half.

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!