The dogbone only lightly hits the shock spring. It doesnt actually lock up the rear diff. I took it outside a few minutes ago and just tried driving it for a few seconds and it might be driving it self by only one rear wheel. Where are the grub screws located so I can check them?
Check the cups that the center dog bones go in. Theres one in front and one in rear. One cup at the back of the tranny then dog bone, then cup at front of rear diff. One cup at front of tranny then dog bone, then rear of front diff. All 4 of these cups have a tiny set screw in them holding them onto a shaft. When they get loose, the dogbone spins the cup, the cup spins on the shaft but doesn't turn the diff.