Edit: Fixed title
This video is a good example of the differences between open differentials and vehicles equipped with lockers.
First Land Rover to go through is a Td5 equipped with ARB differential lockers. At first you see him get stuck and have the wheels spinning, probably because he went in with only hi range and then had to quickly stick it into low range and engaged the Lockers. The next two Land Rovers are older models equipped with only open differentials and a locking center.
Granted the guys with open differentials could have gotten through that with a little more momentum (but would then run the risk of roll over), however I think the point of this video was to show that happens when you have two wheels spinning freely on either end of the truck with open differentials.