Were it me, I'd start by inspecting the universal joints on the rear driveshaft by chocking front and rear of the wheels, on level ground, then putting the transmission in neutral with the handbrake off and seeing if there's any lateral, vertical or rotational play between the driveshaft on one side of the U-joint and the flange on the other side of the U-joint, indicating a bad U-joint.
--Mark
1973 SIII 109 RHD 2.5NA Diesel
0-54mph in just under 11.5 minutes
(9.7 minutes now that she's a 3-door).