look on the tag in your glove box.....If you see a G80 any where it means you have a locked (posi) rear end.
If not... one is gonna spin and the one that has better traction or grip on the pavement isn't.
gear lube is or could be 80 weight "gear lube"
There are two recessed square drive bolts ( just use a 3/8 ratchet and extension) bottom to drain...top to vent and fill. replace the bottom one and then remove the rear cover to fully drain it. new gasket and reseal it. refill the differential to the bottom of the thread level on the top bolt. replace bolt and your done. check for leaks and drive it....