ESP has several parts to it. The four wheel traction control is what is going to help while offroad. Depending on the level of traction available you can get all 4 spinning, or sort of a limitied slip effect where one tire on an axle will be spinning and the other one will spin faster and slower or sort of spin-stop-spin-stop when the traction control activates. It is very dependent on the situation, but always better than no 4 wheel traction control. I have noticed that especially in really slick conditions snow, ice, slick mud etc, that the traction control does a good job of keeping all 4 spinning. Its in the higher traction situations, or when a wheel is lifted off the ground that you get that spin-stop effect.
Of course a locker will ensure all wheels are spinning all the time.
Oh, and I should mention that in 4 wheel HI you will want to push the esp button so that the light is lit up, this will disable the throttle reduction and allow for more tire spin, Of course this is only if you WANT more tire spin.