I never replace bearings unless they show signs of failure. Before a bearing goes, it normally gets loud, then they feel funky as you rotate by hand.
If an inspection shows a smooth bearing 'to the feel' with no lamination or discoloration, why swap it with another that looks the same? Don't get me wrong, at the slightest sign of failure they should be changed.
Certainly, you can change your seals and gaskets without changing your bearings. At the same time, I would use this opportunity to clean out the differential housing and replace two quarts of 90W gear lube. If you want a reason, remove some of the old gear lube into a bucket and look at it in the sunlight. The job will only cost the price of an additional paper gasket and NO shim adjustments will be required. Simply follow your Shop Manual. - Dave