There are 2 areas of backlash to be measured when assembling a diff:
Pinion to Ring gear rotational backlash (needed to do for example when fitting 5.12 R&P to Sierra diff), and
Side Gear to Spider Gear and Housing backlash (needed to do when
correctly installing a Lockrite, etc).
Both measurements, and how to adjust them, are covered in the
Genuine Service Manual chapter on Diffs: