There is a big difference between an AWD and a 4WD, and a big difference between Full time and Part time.
AWD - uses a syncro, hydraulic or clutch style transfer case. which allows all 4 wheels to spin at different speeds but still giving all the wheels a bit of torque in slippery conditions.
This also falls under the fancy computer controlled AWD's as well
4WD - uses a chain or gears in a transfer case to lock the front and rear differentials together, so no matter what there is one rear and one front wheel spinning. This is (according to Jeep) called Part Time 4WD. meaning you will only drive with it engaged for a part of the time.
A Full Time 4WD transfer case is something that can be left engaged, Full Time. It will automatically engage when the rear wheels slip or spin, then disengage after a short period of normal driving. so for the most part your driving in 2WD
My Grand Cherokee has both options for Full and Part time 4WD.
these are basic explanations of how they work. hope it helps.