Trany fluid looks great IMO. I run my level around the same. My next best guess would be wheel bearing/s or like others have mentioned the rear axle seals.
It sounds like you are having to overcome a rolling type force that goes away once it warms up. Try (while it is still cold in the morning) jacking up each wheel one at a time and turning it by hand. That will narrow down front/back, left/right where the problem/s are. I don't think it will be from any of the breaks seizing up because you would probably feel that while breaking (shuttering, pulsing, grinding) and it usually doesn't happen unless the truck has sat for a long period of time. You should be able to turn each wheel by hand once it is off the ground.
Also, is there any sound coming from the wheels while you are having the problem?