Any differential with clutch packs in it requires friction modifier. It's a stinky ass fluid additive. One bottle will do. If you use the more expensive fluid like Royal Purple or Lucas, the modifier is already in it.
The fill plug is on the front side of the differential housing, above where the dogbone bo9lts on. Fill it until you can stick your finger in the hole and can touch the fluid. You can't really over fill it as it will end up spilling out. Basically it gets filled up to the hole.
It's right behind the yellow DOT approved tag.
If you have an aftermarket differential cover, you could fill it through that. But I bet you don't.

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1991 LX 5.0 5Spd
Cobra 4 Wheel Discs/5 Lugs
Saleen SC Wheels w/ 255 Fronts, 275 Rears
Tremec T5
31 Spline Eaton Posi w/ 3.73s
MM Suspension
Stock 5.0 w/ B Cam, Long Tubes, and Pulleys
Dyno Numbers Pending Break In