2006 GMC Sierra whining

Try rotating the tires to eliminate the possibility of a bad tire causing it. Sounds ilike it could be a u-joint or possibly the rear end the way you describe it, especially if the noise changes when you are off and on the gas. Could also be the trans doing it, the trans case and t-case can act as a speaker (aluminum bodies) causing the noise to sound as if it is coming from farther back in the truck. If you can find a dirt road, try in 4wd to see if it still does it or if the noise changes.