Doing a pinion bearing can be done in a weekend, but requires a press to press the old one off and the new one on. There are ways to do it without a press, but it requires the careful use of a die grinder, a freezer and a stove
. Just make sure you keep track of all the shims and bearing caps.
Regearing an axle would take much longer (and would be much more expensive) than just replacing the bearing, due to the finicky nature of setting it up properly, so I would advise holding off on that. Yukon and spicer are good brands.