Being it's a stock rear end, using something different than what ford calls out as far as oil weight and type may ruin it. Friction modifier on the other hand anything aftermarket is just fine. Mine is from Yukon gear, but GM positraction additives the same stuff.
Without it, just turning a corner you will get this bump, bump, bump feeling. That is the clutches in the differential gripping, binding, and releasing multiple times. Friction modifier additive stops that and allows the smooth slippage for turning corners, but full grip when they are needed to grip.
Do NOT get this on your hands. This is stinky assed ****. It will take a week of washing to wear it off. Latex gloves are definitely required. I have to soak the new clutches I have in this stuff before assembling the diff. My shop is gonna stink like **** lol.