Does the whine ever change pitch? If so, does the pitch change go in sync with speed? How about engine speed or transmission shifts? Do those affect pitch or make it go away? How about engine braking with tow/haul mode. When you're slowing using the engine, does it do it?
My first guess would be something in the rear drivetrain if it is speed related. Rear axle or driveshaft.
One other question... are your hubs locked?