i have a 08 silverado and there is a whining noise in the rear end of the truck..i put new tires on it , transmission fluid and checked the u joints...i have checked the fluid in the rear end casing and it seems to be thin could it be breaking down.
Nine times out of ten, or ten times out of ten, this is due to an improper gear setup. Very critical position of the gears is required, pinion depth and and side clearance, to have the perfect mesh required to run quiet.
The housing itself is not quite right and the pinion is running at a slight angle, or something like that. If you can live with it,the rear will probably last forever like this but I'd probably push for a whole new unit.