With the exception of the Off-Road and Pro-4X models all second gens have open differentials.
However, all second gens are equipped with ABLS, or Antilock Brake Limited Slip, which applies the brakes to a rear wheel that slips, sending torque through the open diff to the wheel that has traction. Effectively, this is a pretty decent alternative to a mechanical limited slip differential. Get in some sand or mud, turn off the VDC and stomp the throttle, and you can see it at work. I can not attest to it's effectiveness on rocks, but I know a couple of Armada guys that swear by it.
*4WD Xterras are equipped with 4-wheel ABLS, activated when 4WD is engaged. 2WD Xterras are equipped with rear wheel ABLS only.