Take the diff out of the car and see if when you spin each side diff half that it actually works as a diff. If you spin the left half forward the right half should spin backwards and vice verse.
If that seems to be working, check the bearings in the diff carries and the rear hubs. The bearings in the hubs have a tendency to blow out if you hit lots of things..
IF all seems fine, the last thing to check would be the C-clip that holds the thrust bearing assembly in. That clip has a "flat" side and a rounded side. I have seen them bent slightly, and they would "catch" on the diff half and not let the diff work.
Also I see you have said a couple of times that you have ran the diff "so tight", if you over tighten the diff you can crush the thrust assembly cone washers. That can cause ALL types of problems.
When rebuilding, tighten down maybe a half turn of the screw at a time, then work the diff back and forth a few times. Then tighten half turn, work the diff some more.
ONCE you feel it just starting to tighten up, put a allen key or wrench in each diff half and try to hold them and spin the pulley. You should be able to force the pulley with just a decent amount of force..if its really easy to turn. Tighten the screw only about 1/4 turn try to spin the pulley again. Do this until you can just barley make the pulley break free..
Hope this helps.