If you feel it in the steering wheel, check the power steering fluid level. If you feel it in the seat, that suggests you have the sure-grip locking rear axle and the clutch plates are chattering. There is an additive that is supposed to be used whenever the gear lube is changed. It contains whale oil and other good stuff. The dealer's parts department will have a bottle.
Monday, January 31st, 2011 AT 2:40 AM