"AWD" and "4WD" have different meanings. GMT400 trucks are 4WD, not AWD.
AWD generally means non-switchable, so all four wheels are engaged all the time, usually at 1:1 ratio--no "low range"; and the transfer case typically has a slipper-clutch system or a fluid coupling so that the vehicle doesn't "jump" when turning, and may not send equal torque to front and rear wheels. The system is engineered to be smooth and unobtrusive. The common application is passenger cars and minivans looking for extra traction on snow/ice, or gravel roads.
4WD tends to be switchable, you have the option of selecting 2WD for most driving situations, and 4WD when extra traction is needed. There is "Full Time 4WD", but that tends to be 4WD without the option for 2WD--two ratios in the transfer case (including "low range" for extra power) and no clutch/fluid coupling so the truck steers harder. Torque is split evenly between front and rear.
Another variation is A4WD, the transfer case has a clutch and uses wheel-speed sensors that automatically engage 4WD when the speed sensors detect wheel slippage.
