I believe its pinion related, if you listen, it stays right with the tire rpm. A transmission noise would change with each rpm (gear) change. By moving the lever you are simply isolating the noise from the cab as stated above. Clamp your camera under the bed above the axle and go drive it, I bet the noise stay steady. Get the truck up on stands and run it up in high gear, then slowly pull the park brake to load the gears while holding the rpm steady, it should start to really whine.
Gear noisy will come and go as you load and unload the gears such as climbing a hill or coasting. Pinion bearing noise will stay pretty steady, you will still notice a change but not as dramatic.
I would start at the axle.
Joe
