If it is hard to take off then the diff is welded wrong. Possibly got spatter on the crown/pinion (cue photo from 4WD forum of "welded diff" with crown fully welded to pinion).
For cheap you can get a new centre and weld it properly (though S series centres are a pain to weld) or you can pay the piper and get a T series LSD.
But to reiterate, the symptoms you list aren't typical of a welded diff.