Like the title says. I need to install a new diff cover gasket so I figured I might as well change the fluid (kinda dumb assumption lol) just not sure what to get. What about transmission? 

I use wynns 75-140 synthetic gear lube w/ friction modifier. And for the tranny I use synthetic ATF+4 (dex 4). For the rear end as long as you use some decent fluid and f/modifier your fine I agree with kurt. I use that stuff because its free, lol. But for the trans I like to make sure its something at least quality. Not some off the shelf recycled advance auto chit.
Advance oil isn't recycled. It's just Havoline without a Havoline label. Generic fluids from a parts store are sold the same way Macoroni and Cheese is sold generically at the grocery store. You can look at the SAE label on the back, just like you look at the nutrition facts in the grocery store. You can compare every bottle of conventional oil on the shelf, and they all meet the same SAE standards. None of the oils on that shelf are recycled. Oil is generally not recycled and reused as lubricating oil. Most oil, once it is recycled is used for home heating oil. All the fancy labels is just marketing. It's all basically the same crap, so why would you spend the money on name brand? Advance Auto Antifreeze is just unbranded Prestone. Same stuff, different sticker.
Kurt