Had the rear axle gear oil changed to Mobil 1 synthetic. you can find some photos of it here. http://s1222.photobucket.com/upload?location=1996 Chevy Tahoe 4X4/Rear axle
Existing fluid life unknown as it was in there when i got it in 2010. It is believe to be low on fluid level as well over the past year at least with leaking gasket.
The link didn't work for me.