unless the rear end has been changed the gearing will be 2.73 if its a V8 (l03) or 3.23 if its a V6 (LHO). The RPO codes will verify this, GU5 is 3.23's, can't remember the 2.73 code off hand. Otherwise there should be a stamped code in the front of the axle tube on the passengers side if you can read it... every manual I have says its there, but every car I've checked the diff is too rusted to tell... I should probably check out my convertibles diff one of these days, since its essentially the way it was off the showroom floor.
goodluck. | 