Full time 4WD, also called permanent 4WD, (not to be confused with: part time 4WD ) is a system that powers all four wheels at all times and it can be used full time on all surfaces including pavement. The additional feature of a differential incorporated into the transfer case makes it possible to use 4WD all the time.
2WD is not available (only part time 4WD offers that option). Each tire creates about 25% of the available torque when the ground is level with a uniform surface. Driver has a choice of a "4-high" - 4H (that's your every day setting) and "4-low" - 4L.
Full time 4WD vehicles work very well on-road and are very capable off-road.
When "4-low" is selected the wheels create substantially more torque (on a Grand Cherokee its 2.72 times more) than in "4-high" - at the same time the vehicle moves at substantially slower speeds (2.72 times slower on a Jeep Grand Cherokee).
Important: "4-low" does not create more traction - it creates more torque at slower speeds and that can be detrimental when the ground is slippery. Slipping tires are more likely in "low" than in "high"! On snowy, icy roads "low" would be a bad choice - some really deep snow, however, puts up so much resistance that "low" is needed to push forward. You see, using 4WD is not an easy task.
The low setting is an advantage for drivers who need to tow and maneuver a heavy trailer etc. and for drivers who at one point or another may want to negotiate difficult off-road terrain, when more to