I ran across this outfit that has replacement pans and covers for transmissions and differentials. They increase capacity of both the transmission and differential. The Differential cover that fits the rear end on an f150 gives the rear axle a capacity of 5 quarts. It has a magnetic drain plug and best of all a temperature sender which doubles as a dip stick for the rear end. Being able to monitor the temperature of your differential(s) is really important if you are towing.

http://www.mag-hytec.com/index.htm