ESP is also used to minimize body roll when cornering. Like Nitroman said, turn it off & floor it when you want to break the back end loose. I have about 500 miles on the Nitro in the snow in 4wd & I pretty much leave the ESP alone otherwise.
Going in & out of 4wd doesn't seem to be effected by the ESP at all. If I have the ESP off & the 4wd on, & gas it too much from a standing start in the snow, the truck will spin several tires. With the ESP & 4wd on, the truck seems to lug a bit till the tires can grip, then goes forward. It's amazing how quick the thing is off the line in the snow.