That is a really wonderful question Justin, and I can truly say that it is a good idea to change the oil often.
Use only the same type of oil that came out, for example, if the fluid is red, put ATF type F. The rear gearbox uses a thick 90 weight oil, so be sure that you go for a highway drive and get the gearboxes nice and toasty before you drain them, the sludge will exit quickly.

Before you start, have everything ready, don't work under a jack with out any jackstands.
Get your oil pan and oil sucker along with the appropriate wrenches, always remove the fill plug first, if you cannot, STOP, and do not drain the oil.
Have plenty of rags on hand.

The plugs on the rear axle have conical threads, so the only seal you will need is a TFE pipe paste.

Always replace the copper gasket when you remove any fill or drain plugs, if you don't, oil might spray out or the plug may loosen over time.

Don't use pipe paste on straight threads.