AWD is where the power to the front axle is controlled by a module. If the wheel speed varies by more than a pre programmed amount the front hubs will engage, this can be activated by uneven tyre wear so you should check that they are worn evenly, as I know that the previous owner of my truck had to replace all four tyres after getting an unrepairable puncture in one tyre caused a similar problem when he just fitted one new tyre. Hope this helps.
You can get a better idea of this on snow or wet grass, hit the loud pedal in AWD & the rear wheels will turn maybe 1/2 a turn then you will fel the front hubs kick in. Put it in 4WD (before you stop) do the same again & it will spin up front & rear together.