Guys,
Correct me if I'm wrong but I recently changed my diffs to the aluminum ones and found a single shim on one side of each diff case. The thing was that I had to place the shim on the opposite side of the case when using the aluminum as compared to the plastic one. I made the mistake of putting it back together without changing sides only to find the meshing of gears extremely harsh. The tip to change the shim to the other side came from someone on this forum. Know works like a charm.