Originally Posted by Nothing Special
I've never set up gears myself, but my understanding is that a backlash and pattern check essentially is a full gear set up. If you are re-using your existing gears there's a reasonable chance that you'll hit it right simply by using the same shims / settings that it had before, and the backlash and pattern check is to verify that you're still in the right place. But if those checks don't show you what you want, then you're back into the full gear set up process.
A 'full gear set up" would be done only if you were changing to a different gear ratio. A "check" is just that, checking the pattern by painting the gears with grease and rotating to read the pattern. The pattern is influenced the most, by the pinion gear depth in the housing. You don't have to remove the pinion gear from the housing in order to change the diff. So, pinion depth doesn't change when changing differentials. The backlash is set by side shims at the diff. So, changing the diff only effects the backlash. Bolt the ring gear to the diff, set/check backlash, set diff bearing pre load and a pattern check is done of course just to be sure things are still good.