[RC-Monster Mike]

For shimming, basically you want to eliminate any excess play in the gears. The spider gears mesh with the outdrive gears inside the case. Any play in these gears will cause a lot of stress on them, so you take up the play with shimms (basically small washers). It is important that the gears rotate freely, but there is no additional play. If you can pull or push on the outer shaft (that the driveshaft connect to), then you may need a shim to take the play out. Once complete, the diffs should roll smooth and the outer shafts should not move in and out of the diff. I am not sure if the explanantion is clear enough. LMK if you need further clarification.