check your differential mounting points, that is the easiest of the rear end sounds to fix. replace bushings as necessary. If everything checks out, move on to the U-joints in all three driveshafts. replace u-joints as necessary After that check the differential for slop. You can do this by jacking up one side of the car, and with the car in gear, without the parking brake on, rotate the wheel back and forth by hand and see how much it moves. If it moves alot, then your differential is just loose, as are most z differentials. You may want to check a Junk yard for a tight differential, but they are rather rare around my junk yards.
Hope some of that helped.
-Oliver