I would have test drive it if i were you, may be other problems! If it is not driveable, think i would pass, unless you know the people. No i cant tell you how much to rebuild! As long as motor and trans. are good and you can confirm it maybe, if it has been taken care of otherwise. That is just MY opinion, could be wrong.
rear diff. rebuild cost $1,200.00 parts and labour approx. i would take the vehicle to a gmc dealer and have a thorough inspection done to get a better idea of what u might be getting urself in to. worth the cost of finding out if your lookin at a lemon. the seller should understand.