The awd portion of the 4wd system works slightly different than that. All the power always goes to the front wheels (even the 40/60 sport mode is not technically true). Then off of that the power goes through the ptu, to a coupler, to the rdm. The coupler is what allows slippage for when using it on the road. In 4low, that is actually locked, and that's why you are not suppose to use it on the road as it makes the front and rear tires turn at the same speed. Depending on what mode you set it in (or depending on how much slip is detected), that coupler can clamp down hard or softer, and allow more or less power to go to the rear. The other thing the drive modes change is traction control settings, and throttle mapping, and transmission shift points. For things like sand mode and sport mode, the traction control settings are relaxed and allow the for more wheel slip before clamping down on that tire or reducing throttle. Also, sand and sport will allow the transmission to hold a higher gear so it has plenty of power when needed and to reduce turbo lag. The most split you can ever get is 50/50.
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