Check and see if you have a broken motor mount. There are three, one on each side of the motor and one under the transmission. Another problem could be the carrier bearing for the driveshaft, if it goes out it lets the driveshaft flop around and the rear axle can start hopping, if you don't have the extended cab then most likely you won't have one. Another thing that might cause your symptoms might be the axle bearings, but most of the time you're going to hear them growling or squealing before they get to seizing point. The final issue could be the differential, that would be the expensive part and the most labor to repair. Good luck, hope it's one of the less expensive issues.
