Differential Oil Change and replace with??

Jack both rear tires off the ground and with the car in neutral (front tires blocked, of course) spin the rear tire by hand. If both tires spin the same direction, you have a posi (sure grip). IF when you turn one tire they go in the opposite direction, the it's a single trac, one legger, non posi, or whatever other term there is :D
If it's a "posi", it will need an additive or a gear lube already mixed for a posi.
I'm sure 75w-90 would do you justice.