Well the 63-79 are pretty much the same. The 63-64 are a little different but changing the oil is the same. You need to get a hand pump from a parts store, and suckout the old oil through the fill hole. Get it hot and try to get it level. You can reach in with the right tire off and get the plug off with a long extension.
When I rebuild a differential I always add a drain plug. It's not NCRS but most of the guys would rather be able to do easier maintenance then go for points.
He's right, you have to suck it out. Make sure you go to a GM dealer and get the 4 oz. ($8) posi-additive. You can use GM oil (75w-90) or a good brand like Castrol (2 qts).
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If you have the "X" crossover exhaust pipe running beneath the rear differential (most do), be sure to change your rear end fluid every 10,000 miles because the exhaust heat boils and destroys the fluid. Cheap insurance against a rear end rebuild.
Art
changing rear end fluid
Linear Mode
