The main reason for this is there is less internal friction loss with the front differential disconnected. With all 4x4 vehicles, if the front and rear tire diameters and differential gear ratio's aren't perfectly even, they will get a sort of binding action somewhere between the 2. This robs serious hp even just trying to maintain a constant speed on flat ground, and the more traction you have, the worse the problem is. Also it is common on 4x4's to have a slightly higher (numerically smaller) front axle ratio than rear axle ratio. The reason manufacturers do this is the higher ratio up front makes the front end pull slightly faster than the rear making it almost impossible for the vehicle to swap ends under power. My '91 F-350 4x4 has a 4:10 ratio in back, and a 4:09 up front, and it's very stable even on packed snow and ice because of this. This different ratio thing causes even more binding in good traction conditions, in fact that's why most 4x4 trucks recommend only using 4 wheel mode in slick poor traction conditions, the binding caused by using it on bare pavement can destroy anything in the drivetrain including differentials, U- joints, and even transfer cases. I hope this helps you understand a little bit about why you are making it up the hills easier, and why you have more power in 2 wheel mode.
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