I have a '91 W250 that has recently started to vibrate excessively at low to moderate speeds. It doesn't appear to be caused by any misalignment because when the clutch is engaged the ride smoothes out. When I rev the engine while sitting I can't detect any obvious problems with the motor. Any ideas what I should check to diagnose this problem?

 

How menny miles on it? I would start with you motor and tranny mounts.

 

get tires alighned maybe. or check drivetrain ie. driveshaft, ujoints,tranny.
just a thought.

 

Vibrate where? In the front end? The transmission? The driveline? My truck vibrates all over!

When you say it smoothes out with the clutch engaged do you mean with the pedal down (disengaged) and coasting? If so you may have a pilot bearing out or the input bearing to the tranny might be bad.

Please fill out your sig so we know what you have and any extra bomb's you've added.

Edwin