If your diff balls are carbide or ceramic you can get away with with a second use as long as they were treated properly the first round. As in broken in correctly and no slipping (barking). Most of the time the grooves that get cut into the rings are what give the gritty feel but mostly the thrust bearing is what takes the most abuse. Sanding the rings is not recommended, you can polish them on a sharpening stone with cutting oil but it's not worth it as the balls cut their own groove anyways. For $28 per diff rebuild, really your not saving much by reusing anything and even then you won't have a completely smooth diff compared to everything being new.