The answer by Just-In is incorrect.
You may have a limited slip rear differential; that is an option with either 4x4's or 4x2's, so you can have it or you cannot. If you do, the wheel with traction gets the power (the wheel with the MOST resistance, that is); if you don't, it's the opposite.
The front differential, when in 4x4 mode (either H or L) will transfer power to both front wheels simultaneously; no slip differential here. That is why you should never use 4x4 in dry pavement, you will granade the front differential in the first turn (when the wheels travel at different speed).
Hope this helps.