i had the same problem in a 2012 4wd. rythmic humming in 2wd that went away when i engaged 4wd. After chatting with Dan at 4 Wheel Auto, he told me to watch East Coast Gear supply's video on diff bearing replacement. i did watch. they described the problem accurately. in my case it was indeed the needle bearing on the driver's side of the diff. I replaced it with their bushing and the humming stopped. i have a spare bushing in the garage.
My truck has OME 886 front and dakar rears, with the diff drop in front. i have other front end issues, but the one you describe sounds a lot like what I tackled and fixed. as the other posters noted, it could be something else. good luck.
Chris