I think I figured it out today...had the sliding rear window open, but not my side windows...
I could hear it, and it changed with speed....has me thinking Rear Differential.
<2400miles on the odometer right now, and I'll probably get in w/ the dealer tomorrow and take the guy for a spin and see what he says. The only other thing I can think of is it could be u-joints (not very likely, since this sounds like gears meshing at high speed), or the transfer case, which I'm not used to a transfer case. (1st 4wd). Truck is an automatic, and I've taken in and out of gear, so that rules out at least the front half of the transmission forward.
That said, I have another high pitch whine that comes from the front of the truck, I'll have them look at that while I'm at it.