I would agree with a proper inspection of the driveline to be sure the flex discs, u-joint and center bearing are all doing well.

If so, then zip-tie a smart phone (or other small recording device) near the differential and drive it into the range where the noise appears. Then do the same with it zip-tied near each wheel. That will help you track down the noise.

Does the noise change in frequency (pitch) as speed changes? Wheel bearings are usually more of a cyclical noise (sort of a whooop, whooop, whooop) than constant. Differential bearings and gears can make an interesting assortment of noises.

Still, I'd start with the external stuff that can more easily be inspected, then go on from there.

Good luck.