If the rearend had 100,000+ miles on it the pinion would not have the preload that people think it should. But that's just wear and tear on the bearings and would spin free. Pinion preload has no relation with backlash unless the pinion nut is backed off, but at that point it would be a noisy oily mess. I would not worry about the pattern on the used gears if it was not making noise it's good. If gears were set up improperly and someone put 50,000+ miles on them you will never get the proper pattern and would most likely make noise.
