Some rear differential carriers are worn to the point where the carrier needs to be shimmed to get the correct pre-load set. Bratton's carries the shims for this. Before I learned of this, I was doing a rebuild and ended up with a huge amount of gaskets on the axle housings. Another good reason you should measure the pre-load and adjust if necessary.
http://www.bestgasket.com/drive_trai...0U-Joint%20Set