both the fill and drain plugs are on the side of the halfshaft, the drain plug is not on the bottom of the diff. both are facing the front of the truck, look for them, the sway bar and other steering stuff could be in the way so you don't see them, thats why to fill you need a pump. remove the spash gaurd you should be able to see them and get to them.

2001 5sp. K&N stock filter 215/70/15
If your check engine light is on at least you know the light still works