If the rear axle bearings have not been making any noise, I would not mess with replacing them, especially if they do not look/feel bad. Change out the seal, and you would be good to go. Tape up the openings, as to prevent any slag or other junk to get into the bearings, but if they are not broke, there is no reason to mess with them, they usually last for a long, long time.
I would follow Posers advice. Don't pull the bearing unless you have to. I had the 3rd member out of mine and thought I'd go ahead and pull the bearings as well as the seals. I mangled the first one w/ a slide hammer type puller (don't advise this method). For the other side, I pushed a long length of pipe through the entire axle housing and rammed them out (much easier and saved the bearings). These bearing are hard to find. I had to call many bearing shops in Portland in order to track'em down. They may be more readily available through a Toyota dealer though.