a locker is a type of differential, a locking differential allows both tires to move at the same speed, pretty much theyre locked together, they allow maxiumum amount of traction offroad, so if you have an non locking, open differential on your X, the tire without traction is going to spin, so if you are in an off camber spot, and one of your rear tires leaves the ground, the the one in the air will spin, leaving the one on the ground with traction useless, with a locker, both tires will move in the same direction at the same speed, so back in the off camber situation, the tire thats in the air, AND the one on the ground will spin, resulting in you moving forward or backwards out of the spot...

http://auto.howstuffworks.com/differential10.htm