The free wheeling hubs disconnect the from wheels from the driveshafts - supposed to help with fuel economy as you're not draging the front shafts and diff around, though interestingly (or not) Nissan deleted them from the very last of the Terrano 2's so with focus on CO2 emissions my bet is their testing showed no real gain - I might bewrong though.
Most newer jap trucks have auto hubs, you jut have to reverse up to dissengage them. I leave mine enganged all day when I'm out green laning and as you say just shift from 4x2 to 4x4 and back as appropriate and then I dissengage them when I'm done for the day - Thats what I'd advise you do with yours.
I assume you know not to drive in 4x4 on ard dry surfaces or you'll end up breaking something in the transmission.