Nearly any GL-5 or GL-6 gear lube is fine and personally, I use and recommend a NON-synthetic gear lube for the axles as it seems to help them run a little cooler than a synthetic does.
And if your rear axle has a Tracloc limited slip differential, you just need to make sure the back of the bottle says something like "compatible with limited slip differentials" as most of them are. 70W-80, 75W-90 etc. is fine for cooler area, 80W-120, 85W-140 etc. is fine for towing, and/or warmer climates. The axles are not especially fussy about the exact viscosity gear lube they get. You'll need two quarts but it doesn't quite hold all two quarts, something like 1.8 so you'll have a little left over. If you don't have a Tracloc, you can still use that type of gear lube as it is compatible with non-Tracloc equipped axles too.
Regarding sealing the differential cover back to the housing goes, Permatek Ultra Black RTV (like silicone) works fine and is leak free if you don't overtighten the bolts so much that too much of the RTV squeezes out. Instructions for its use are on the tube and pretty much any auto parts store should carry it.