My diff is totally slipping and I am unable to tighten it up. I pulled the whole thing out to have a look at it and not matter what I try I can't undo the diff screw. Does anyone know why and have any tips for fixing the problem?

Truck is two race meets old as well.

Check ur diff nut (the threads the screw goes into) and also check the thrust bearing. If you didnt grease the bearing or u put it together wrong your diff will not stay tight while running.