Drain and fill the rear axle housing every 100K miles or after fording any streams.
Use hypoid gear lubricant SAE 80 or 90. 90 if you beat on it and/or it is a little noisy, Mobile One makes a nice synthetic (just make sure it is the G5 stuff found on the small print on the back lable).
If you have a T-lok, 4 oz. of Ford Friction Modifier C8AZ-19B546-A or equivalent.