I have a 99 Chevy Tahoe 4x4 with a standard rear end- going to replace the rear differential fluid. How much and what type of oil do I use. Any additives???

It should take a little under 3 quarts of fluid. May want to have 4 on hand just in case. All NBS trucks have synthetic axle fluid (I think!), but you don't need to use the expensive GM grape juice unless you have the Eaton G80 locker, which I assume you mean you don't when you say standard. Any synthetic axle fluid is fine, if I didn't have the locker, I'd probably use M1 75w90.