Morning after a 3+ hour drive truck started to make whining noises. Noise comes from driveshaft area, towards the back near the diff. Sound is at any speed/gear, gets higher pitched the faster the shaft spins. I will try to upload a video of it.

 

Could just be very low on fluid?

I would drain the differential fluid and have a look at it, see if there are metal filings in there, and remove cover to visually inspect parts and to change the gasket.

U-joints will make a clunking sound when there is a speed changed and I didn't hear that.

wheel/axle bearing going out would usually be louder on one side and you didn't mention that.

 

I'm having the same issue in mine.
What would make the differential fluid run low? I'm not very mechanically inclined, so I will probably have to take mine to a mechanic, and I would kind of like to have an idea of what's going on before I get in there.

 

I had someone check my rear diff, and the fluid was a little low, but not bad, but the seal between the rear diff and the U joint was blown out, so I still need to take it to a mechanic...