http://www.pps.net.au/4wdencounter/a...difflocks.html


"A 'normal' differential transmits drive to both wheels equally when they
receive equal traction, but when one wheel loses traction, the diff actually
transmits more drive to that wheel. Hence you can become stuck in 2WD with
only one wheel bogged. Similairly, when in 4WD you can be bogged with one
front and one rear wheel without traction. So your 4WD is really only a 2WD,
with one front and one rear wheel providing all forward momentum."