You unscrew the plug on the front cover, and ad 75w90 to the bottom of the hole.
Just fill it until the lube runs out the fill hole. It is now full.
It is in the center of the differential cover.
It is a rubber plug in the front cover.
Driver's side of differntial, mid-way up case. You'll see a pipe threaded plug.
There is a plug on the front of the case. Usually a 3/8" square socket plug. Could be dirty enough not to see. Keep looking!
Through the check plug hole in the rear of the cover.
On the front, about 3" up. On mine it was a 24mm nut.
Usually you fill it to the bottom of the hole.
there is a drain (on the bottom) and fill plug (toward the top) for the front differential. 1.6 quarts of 75W90 GL-4 or GL5 gear oil.
Remove the fill plug from the 2001 Explorer differential. Remove the cover from the head to allow the fluid to drain. put the cover back on with a new gasket, then fill the unit up through the fill hole.
3 1/4 qts. or fill until the oil starts to come out the bottom of the fill hole.
The fill plug is on the rear cover of the differential. Remove the rear cover to drain the fluid.