Like I said rear bearings are a press fit which bolts to the housing ends so you do not loose a wheel when you break an axle (a very common occurrence with the old 8.2 inch diameter ring gear ten bolt differential). Because it requires a fifty ton press most car owners do not have one (or the tooling needed to change the bearing and install a new seal). A machine shop will charge you for your lack of fore sight in not buying the press to keep in your living room, for such an application, but at least the wife will be happy (if she questions the need for that added expense you could mention your desire to have a hydraulic press of your own for such tasks stored in the living room).
I feel new bearings are a good investment if they show signs of wear. I would pull the axels and check them visually for signs of wear (galling, or bluing, or excessive movement in the cage. You should also notice a matching groove in the bearing race if the bearings are bad. You will need a new paper gasket to reinstall the axels if the bearings are still good, and 2-1/2 quarts of SAE 90 weight hypoid grease.