Because your Denalis are full time AWD they will (and I hate to say this) have this noise.
It IS normal, and there are no fixes. Well, I guess you could replace the front axle with a standard 4WD Yukon axle and install a switc and have the computer replaced and............ You get the picture.
Unfortunately, the sound will stay, until GM engineers a differential with no gears in it.
We have replaced driveshafts, transfer cases and in one case an axle assembly to try to keep customers happy. All, by the way, with no fix to the "problem".
