It could be any of a number of bearings, most likely one of the wheel bearings but you can't find it that way. Spinning one wheel at a time engages the side gears in the differential which creates more noise that any bearing would. The noise will also be quite different without the weight of the truck on it.

Thank you-- I might be looking at a bad wheel bearing if not both.

Yes, but it could also be coming from the differential. You need a pro to listen to it.

Thanks--It ended up being a bad front pinion gear bearing.

Yes, that is the possibility I was concerned about. A trained ear can tell the difference between that and a wheel bearing. Glad you found it.