We had a Chassis-Ear at work that had 6 magnetic microphones with long leads that we could attach to various underbody components and play through headphones or the vehicle's stereo. We could select which mike to listen to. It was a great diagnostic tool.
Someone sitting in the bed of the truck may be able to tell whether it's a left or right wheel bearing or in the center (differential). A howl certainly sounds like a gear noise. Has the hypoid fluid been changed or the rear cover ever been off? A synthetic 75W-140 may quiet it down. Sometimes you can visibly see the scuffing on the gear teeth.