How do you drain the rear differential fluid on a 1991 F250?

Question

I think I see the filler plug but I don't know where to drain it from.

Answer 1

You pull the differential cover. Start by loosening the bolts on the bottom and work your way up. You may need to use a flathead screwdriver to pry the cover off. Be careful to not bend the cover.

Answer 2

If you can't locate a drain plug, there two ways to doing it.  One is to remove the drive line and the oil will flow from there and the other is removing the rear cover.  

I would remove the rear cover, since you can clean out any crud inside and get to see the condition of the gears.  You'll need new gasket or use high temp silicone when installing it.