Sorry for the late answer, but, per the owner's manual, ABLS (Active Brake Limited Slip) is always active. It does not turn off when VDC is turned off. It does not turn off in 4LO. In 2WD mode it works on two wheels, in 4WD mode it works on four wheels, or, more accurately, it works on two pairs of wheels.
It is active IF you are going above 3-6 mph. It's not for rock crawling.
http://www.manualslib.com/manual/458339 ... l?page=366
I exercise mine regularly, in 4LO, and I can hear it engaging - the RPM will rise a bit, then drop, as a wheel spins for a half second, then is braked.
Two weeks ago I climbed a particularly nasty hill of loose rock, bounced all over the place but it never stopped. My buddy followed me in his Wrangler, got stuck halfway, spun his tires then snapped both rear axle shafts. I had to go back down part way and use my car as an anchor point to winch him to the bottom.
I watched a 2WD Xterra going up a hill and I could see his tire spin, grab, spin, grab - he had no idea ABLS was doing its thing and keeping him from getting stuck. Pretty clever system.