The difference between 4WD high and low is a gearing ratio, usually. If you put it into low, the vehicle will go slower for a given level of engine revolutions, and that's a pretty basic explantion. It allows the vehicle to have more torque to run at lower speeds.
My guess is that the absolute limit for depth of water is determined by wiring. Allow your wires to get wet, and the ignition may not fire because the spark will run to ground before it gets to the spark plug. But that's a guess.
Edited by Dan_McI (02/13/08 08:26 PM)