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Some other considerations other than gas mileage:
AWD systems (like Audi's early quattro before all the electronic shenanigans and Subie's AWD systems) are based on feeding power to a center differential which in turn sends power front and back. There are a few vehicle design attributes that make this a good fit on cars: All the wheels are generally on the ground at the same time / limited body twist, you don't have extreme turning angles, and vehicle load front/back is relatively constant.
Pickups have some load cases that occur more frequently. You're frequently getting into larger steering angles which can cause binding in the front diff and force wheel slip (I get this frequently when in 4WD with my truck on slippery surfaces). Trucks, when off-road, may tend to lift a tire on one axle; the transfer case (with no center diff) will still still be sending power to the other axle to pull you through that while the axle with a lifted tire is spinning a wheel. A center diff may just bias all the power to the spinning axle. (Yes, you can limited-slip center diffs and front/rear diffs, but these tend to wear quickly and have high service requirements on pickups.)
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