Car trouble

If it's a high frequency vibration, check for a tight u-joint. Look for signs of a reddish rust around the bearing caps, otherwise you'll have to unbolt it to check them. If it's a lower speed shaking, an engine misfire condition is more likely.

Don't overlook a drive line vibration from incorrect ride height, usually due to sagged rear springs. That will reduce the angle of the drive shaft to the transmission's output shaft and the pinion gear in the axle. There needs to be a little angle there. An alignment shop will have a small book that shows where to take the measurements and what they should be.