Backlash and preload are important. Backlash should be .008-.010, Preload is also jut as important, but there not exactly written instructions on how to get proper preload. You need the shims to go in tight, and you should not be able to "pull" the diff case out by hand, once its in all the way. It should take some effort to remove it with say a 4' crow bar or pry bar. Depending on the shims, most are cast iron and break easily. It takes practice to get them in without breakage. They sell aftermarket ones that allow you to "stack" thinner shims between thicker outer shims. I don't use the aftermarket ones unless I have no other shims to use. The factory outer shims come in .002 increments. Hope that helps you.
