It is possible for the driveshaft to become unbalanced, if one of the weights comes off. The more likely candidate is that the u-joint has become notched. The notch in the joint is very hard to feel by hand. This will give you the feeling of an out of balance. The U is a pressed in piece and is not user serviced. A high quality driveshaft shop can balance a MB shaft, but these are few and far between. Pressing in a new piece will cause a nice shaft to be out of balance Better off buying a new (rebuilt) driveshaft. It comes with a carrier bearing, and is not terribly expensive.

The MB shaft of this era comes apart just in front of the carrier, at the nut mentioned in previous posts. This is a splined piece, that if put back together one spline off will cause a true unbalance. The shaft is balanced and weighted as a unit. If you are taking apart the shaft, take a piece of tape along the axis across the spline (across the nut) and then some spray paint on the tape in order to get a nice sharp line. It is imperative to replace at the exact spline.

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Mercury is a boxer

1984 300d
1999 Prelude
Medici road bike
Klein touring bike
Klein mtn bike