Sounds like you are tightening it to much to be honest, you may well have damaged the balls or the diff plates
I would get some new ceramic balls & some new plates then do this
Assemble diff but do not do the screw up tight, put a screw driver through both outdrives & hold them so they cant rotate, then see if the centre gear will turn by hand, if it does it is too loose, so tighten the screw a quarter turn, then try the centre gear again, if still turning by hand then tighten another quarter turn, check again & again & again until you cant turn the gear by hand, once you can't turn the gear it should be set for it's first run, remember the diff will probably need re tightening once it's done a few laps
Make sure you use plenty of black grease on the thrust balls when putting it together
This is a good guide to explain what i just said in pictures
http://www.youtube.com/watch?v=IIYPo...layer_embedded
http://www.youtube.com/watch?v=Ct1CU...eature=related
Hope that helps
