I have never really looked at my rear, but I just turned 75K and want to switch to synthetic fluid. It is a posi so what can I get local that would work the best...

 

Also what is hte best way t odo it? Remove the whole cover or just crack it so it can leak out. Plus how do you fill it back up?

 

Suitably raise the vehicle so that the rear axle is in droop.

Remove the bolts from the rear cover, leave a top bolt loosely threaded on. Place a container and newspaper underneath the rear. Gently pry the cover off and the fluid will drain out. When most of the fluid has been drained remove the last bolt and the cover. Let the fluid drain while you clean off the magnet and rear cover. A solvent like carb cleaner makes quick work of squirting the old gear oil off of the cover, gears, etc. Make sure the surface between the rear end case and cover is clean and smooth, install new gasket or RTV, and bolt the cover back on (22 lb. ft.).

The fill plug is located on the right (passenger) side of the rear end housing. To remove/install the plug you use a 3/8 inch ratchet without any socket, just the square drive. Filling is a pain in the neck. I use a container with a hose to squeeze the fluid in. Getting the gear oil warm will help. Always add a bottle of GM LSD additive first, followed by two quarts of gear oil.

I think that covers the basics

 

A couple of minor comments to above:
According to my manual, use a gasket and put RTV on the cover side of the gasket.
You won't get two quarts of gear lube and a bottle og GM posi additive in there. More like 1 1/2 qts.
Have plenty of rags on hand so you can wipe out the insice of the cover and diff. It's kind of messy and smelly. Gloves would not be a bad idea.