There are locker rear ends like the Detroit locker which completely locks up both rear wheels and unlocks when it senses you turning a corner. They are great for drag racing but annoying (noisy) and potentially dangerous if a novice is at the wheel. Hitting a wet spot under power can turn you around. Then there is limited slip like the Detroit True-Track (which I have) which does not completely lockup but gives a posi-traction-like delivery of traction when it senses tire slip. The GM positraction and the Ford Track-Loc rear ends use discs to provide traction to both rear wheels while the True-Track uses worm gears. Each has its advantages and disadvantages. Choosing the one that is right for you depends on your vehicle and driving habits.