Jack up rear wheelS(both). Put in neutral w/emer. brake off. Have someone hold one rear wheel still while you try to spin the opposite one. If it spins ok, it's unlocked; if it is extremely difficult to turn, or will not turn, it is locked. Had an acquaintance lose his responsibility for ordering new cars when he ordered all GMC 4X4's w/o specifying locked differentials. Perhaps even a 2X w/posi-trac'n. differential will pull better than a 4X w/unlocked differentials?! s