Your diff should not be tight. You want it as loose as possible without it slipping (or else it can really make your handling screwy).

Other causes could be bad bearing, or improperly seated bearings (ie, not fully in the ride heigh adjuster or the adjuster isnt properly seated in the bulkhead, or the adjuster needs to be reamed out since its too tight for the bearing).

And just to make sure there is nothing wrong I would have someone else look at the car, since sometimes you dont notice small things that can be wrong.