There are a couple of possibilities on your truck, none of them simple. If you are hearing a whine, listen to it and is it worse when applying throttle or off throttle (coasting)? If the noise changes with throttle setting (on or off) then the ring and pinion are bad and the differential will need to be rebuilt.

Is the noise louder when turning a corner at higher speeds (above 20 mph) and not a sharp corner, say an off ramp? If the noise is louder on right turns then your left wheel nearing is bad and chances are the left axle is also damaged. And on left turns the same with the right side. If the axle bearings are bad the differential cover will have to come off to pull the axles out.

If you have a chatter when turning tight corners (say at parking lot speeds) and while appling throttle, then the clutches in the differential are bad and the differential will have to be disassembled and the clutches replaced.

If you hear the whine all the time reguardless of load and you are certain that it is coming from the rear axle area, then the side bearings on the differential are bad and the differential will have to be rebuilt.

And last but not least but really should be checked first. Check the tires, if they are badly cupped and or worn then you might want to rotate the tires to the front and see if the noise changes pattern.

This is about the best I can do without actually driving the truck. A lot of this type of diagnosis really requires a technician to drive the vehicle and put it through several tests. First of all, our ears are tuned through experence as to where the noise is coming from and what could be causing it. After that it is all a matter of physically inspecting the vehicle for possibilities.

Let me know.