99 Silverado 5.3L 4X4 Problem

my 99 silverado makes a loud humming sound when i put it in 4 wheel drive. i took the truck into the dealer who immediately recommended new bearings which i agreed upon, but even after the new bearings were installed it still makes the same humming sound. the dealer told me it was aboslutely fine and that there wasnt any mechanical problems that would be causing this. as well when i take the truck out of 4 wheel drive, the wheels stay locked in until i release the gas, then once i tap the gas pedal again they release. now this humming noise is not quite, it is extremely loud, i can hear it in the cab over my 2 10 inch subwoofers and its really bothers me.

does anyone have any information about this problem ??