Most modern cars and light trucks have a differential. All-wheel drive vehicles need a differential between each set of wheels as well as a differential between the front and back wheels because the front wheels travel a different distance through a turn than the rear wheels. The differential transfers engine power to the wheels and adjusts to the difference in the wheel speed.
Transmission fluids deteriorate over time and driving conditions can play a role in how often your automatic transmission fluid should be replaced. Stop and go city driving, hauling heavy loads, and towing a trailer can accelerate the deterioration. These driving conditions can raise the operating temperature of the transmission.