Hi (this is as far as i have determined but i will confirm when i have had a better look) i think when the 2wd is selected the front prop stops turning and the front axle disconnects from the drive shafts using an internal clutch very similar to the shift on the fly system on the 'rodeo' US or 'Frontera B UK . so the outer CV joints turn the drive shaft and the inner joints but the diff doesn't turn because the internal part of the drive shafts are disconnected using a clutch type mechanism - which means that the front prop doesn't move either - its all very complex and so much to go wrong with sensors etc
if it is the same then i can change from 2wd to 4wd at any speed up to 50mph the same as the shift on the fly system as fitted to my wifes car but i shall be having a really good look in a few weeks time as i will be doing all the fluid changes
cheers
Steve