www.attrelltoyota.com/pdf/04highlander_e.pdf
Highlander 4WD v6 features a full-time 4WD system that provides driving force to all four wheels at all times in normal driving conditions. Over rough, sandy or wet roads, where traction is variableor unsure, the system works to transfer power when one wheel starts to slip; pumping power to the other wheels.
My friend who bought a 2002 Acura MDX said it's different
on his MDX, he said the MDX is a part-time 4WD , meaning in normal driving conditions, only the front two wheels get power, if slippage is detected, then power is transferred to the rear wheels. Unfortunately, he found this out the hard way when he got stuck in the snow, he hates the MDX and is gonna go with the new Jeep Grand Cherokee instead.