I was going to point out what Dvdswan said. In my situation I would use the brakes to stop moving. Still holding the brakes, shift into reverse, start to back up and clunk. When I hit the brakes the nose dived which extended the rear drive line at the slip yoke and when I finally came to a complete stop the slip yoke was bound up tight. (Still holding brakes). As soon as I let off the brakes the binding let loose and the slip yoke went back to a non binding neutral position. That bind can happen with a slip yoke or even the slip portion of a standard shaft with the slip built into the shaft. Might try pulling the rear driveline to eliminate it as the culprit. Ive also seen the drums move on a semi float rear. Hope this helps and/or you find your problem. Good luck.
