The only thing that was offered from GM in these rear ends was the cone type Auburn and the Governor Lock. The late '90s F body cars got the Torsen, but these were all 28 spline. The Auburn and Gov Lock are junk and should not be used in a serious performance car, but then neigher should the 7.5 rear end. The best replacement differential for the 7.5 is the Eaton Posi or Duragrip. These are after market, so you won't find one in a junk yard. If you find an Auburn then most likely it will already be worn out. They can't be rebuilt, so they are scrap metal. The S trucks only got the Gov Lock, which will break if you abuse it any. Then you will be spinning one wheel again. You are better off just getting a good Eaton or Duragrip and be done with it.