What fluid do I put in the rear differential for my 01 Ford Explorer Sport?

If you can read the tag on your rear differential cover it will tell you there.
But just in case it's rusted beyond visibility here is what you need.
A traction lock (limited slip) differential takes the synthetic 75w140. You may need some of Fords limited slip additive to prevent the clutch plates from grabbing as well.
Conventional differential takes 80w90 no additive required.
If you are not sure what yours is jack up the rear end, support it properly and block the wheels to keep it from rolling..
If yours is 4 wheel drive make sure it is disengaged and put it in neutral.
Do not start engine. Grab one rear wheel and rotate it. If the wheel on the opposite side rotates the same direction it is traction lock