Bill,
Sorry I did not reply earlier. There are so many things back there that could cause a knocking sound that only a thorough visual inspection may be the only true way to know. I would start with items such as the dampers and damper links to make sure they are attached to the frame, grab the driveshaft and give it a good shake for any improper movement, check springs for broken leaf, wire wheels? check splines for wear. If these prove good then your next best look should be in the rear axle. You will have to remove the rear cover. knocking noise from the axle may include splines on halfshafts or in differential gears being badly worn. Improper shimming of gears, teeth damaged, gears worn, or worse gears broken off. Based on what I hear about taking the rear axle apart it sounds like it can be a real bear of a job if you plan on doing it yourself.