Does the posi still work OK? Additive will not fix one, in fact it makes them slip more if they are worn out.
Use a heavy oil if its got a ton of miles, or recommemded weight oil with a normal amount of miles. One bottle of GM or NAPA posi additive should be all you need. 80/90 would be about right.
Hit a open parking lot and do a bunch of real slow figure eights to work the oil additive into the clutches. Dad used to do this after long trips on the highway in our old wagon. The first turn or two off the highway sounded like the axle was breaking! A few figure eights and it was back to normal.
Joe
