I have to admit I jumped in with my comments looking to hearwhat came back, The 99 SS I just picked up has a differential whine from 45 to about 60 also.Not bad but there. There are only 18,000 miles on it and I have not checked thefluids yet. Since I saw someone else with the same issue I jumped in. I apologize if i'm beating a dead horse. Mine is not load enough to concern me atthis time but I know it’s not perfect. I’m figuring the previous own just didtoo many burn outs during break in. Any thoughts? Thanks for any input Old Timer

 

If it's a whine, it's like ring/pinion, but if its more of a whiring noise that varies with speed it's more likely to be diff bearings...

Definitely try a fluid change first to see if that helps things, and don't forget the additive.

If you are looking for a shop, you may be well-served by visiting a specialty shop, such as this one which I have visited:
http://www.suburbandriveline.com