Shims go on the outside of the bearing race; between the race and the end of the axle tube. You need to shim the carrier so that you've got the required backlash in the gears (usually etched on the side of the ring gear). Measure the backlash with the bearings lightly loaded, in other words with the shims so that the last shim goes in light resistence. After you've gotten the proper shimming to achieve the backlash, add 0.006" to each side to obtain proper bearing preload.
Hope this helps.
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Jeff Chambers
T/S #3
11.86 Seconds/113.8 MPH
Chambers Racing Team