What gear is in it now? If you have a 2.14 to 3.08 gear then you have a 2 series differential. If you have a 3.23 to 4.56 then you have a 3 series differential. If you have a 2 series you can get a thick 3.73 gear that will work on the 2 series without a spacer. I do not recommend using a spacer, especially in the 7.5 rear end.
Look at post number 8 in this thread. 10 bolt GM Rear End Rebuild