Re: Rear Diff Fluid
Depending on the distribution network WalMart's SuperTech oil is either Mobile Oil or Warren Petroleum Products (look on the bottom of the bottle it'll say WPP if it's Warren). Is it synthetic? The Sterlings spec synthetic oil, which in the case of most gear oils means it'll be a group III oil. For OTC non-special oils RedLine makes some of the best gear oils you can get.
For less expensive options Schaeffers makes great gear oils and saved a lot of our failure prone diffs at work. Also, Delo's ESI line of gear lubes are great for the money. I think they are one of the only "mineral" based gear oils to get the 750K mile drain cert from Eaton.
Add very small amounts of friction modifier, too much is bad. It's easier to add more, not so easy to take it out.