when is it necessary to shim the diffs?

[_paralyzed_]

bb16- you meant pinion and ring gear, not spur, I got your back! Shimming makes the difference between stripping a gear and not stripping a gear. You need the cases open anyway to change oil, i would absolutely shim if you can pull the pinion in or out at all with the case closed, as much as you can move it is as much slop that you've got, basically room to fail. Conversely, too tight a mesh will cause heat and failure. It's trial and error getting a good mesh, but is well worth the time spent.