Just found this....
http://www.jeep.com/en/4x4/faqs/ Quote:
What is the difference between Full-Time & Part-Time 4WD systems?
Full-time 4WD systems utilize a center differential, which enables the front and rear driveshafts to turn at different speeds, thereby allowing engagement on dry surfaces for normal driving conditions.
A part-time system does not employ a center differential and locks the front and rear driveshafts together. With a part-time system, 2WD mode should be used during normal driving conditions and 4WD mode is to be used only when off-road or on wet or slippery surfaces.