Really shimming the gears inside the diff is just a case of putting together and check mesh for minimal backlash on gears by hold one outdrive and turn the outdrive on the other side and see how much it moves anticlockwise and clockwise. You must also twist each outdrive through a complete turn checking for binding. If you feel too much backlash then add shims but always check for tight spots on a full revolution, If you feel any tight spots then you need to remove shims. So it is a case of trying to reach a happy medium of the minimum amount of backlash without and tight spots. Also as the diff gets older then you may be able to add more shims due to wear on the gears but you just keep doing the same process over and over again when you rebuild the diff. Also make sure that when you are tightening down the screws on the diff to make sure that you get the screw head to meet the plastic and then only tighten it up maybe 1/4 of a turn to make sure that you are not deforming the plastic which could mean you end up with a tight spot in your diff. This goes for any screw head that meets plastic especially in and around the gearbox as the screw head should just meet the plastic and then be tightened up a small amount. A typical example of this going wrong are usually the screws that go through the rear gearbox and into the motor mount where I have see people over tighten and then wonder why the motor has overheated. You can tell as there are many hard spots as you turn the spur gear because everything is now a bit deformed and the bearings are being crushed again the layshaft, idler gear and or diff case causing binding.

On a non belt drive 4wd you can shim the diffs left and right of the bearings on the outdrive of the diff as well as adding or removing shims just behind the pinion gear which meshes with the ring gear as well. In a 4wd diff the bearings that hold the diff and especially the bearings the are holding the pinion must be in very good condition otherwise under load the mesh can change with disastrous effects.

A lot of this is trial and error.