S6S - are you talking about the 4 small shims? there are 2 big shims that go behind the 2 big gears in the diff as shown on the manual page that Eddie posted - but there are also 4 small shims in mine that are not shown on the manual page - those 4 go behind the 4 small gears - it sounds like you are thinking that already

On my RXT, I bought some extras of the big shims so that I can adjust the mesh of my diffs - initially, the mesh was loose, and I quickly wore out the teeth on the little gears - and the metal dust in the oil really ate up the case behind the little gears - if I remember correctly, I added one big shim behind just one of the big gears (3 total big shims in the diff), and they are lasting much longer