Both the above are correct. Your rear posi will always act as a LSD. In 2wd your front is just an open, unpowered axle - there's nothing to slip because there's no power. When you're in 4wd, now there's power and it will also act as a LSD.
Utah Jeep Crew #148
No powerball here? How am I supposed to finance the mods?!?
'12 Sport...miss my YJ for all the wrong reasons though