I am considering purchasing a 2017 GMC Denali 2500 and experienced this same sound when in 4WD Hi. It sounds as if you can hear the gears turning, but fighting the resistance of the load both while accelerating and decelerating. No noise in 2WD. My brother has a 2500 Sierra and he doesn't hear any noise in 4WD.
Has there been any resolution to any of your issues? Or was this deemed normal?