8 3/4 dif

I just check the fluid when it's warm.If it's not brown and foamy at all, then there is no water in it. If it smells like gear oil is supposed to smell like, then the fluid has not been overheated nor overworked. If the level is good, Then I'd be inclined to leave it be. I'm lazy like that.
The fluid in my diff is circa 2002. It has 125,000 or so miles on it. It smells a little fishy, but it's supposed to,lol.