A transfer case is a gearbox that transfers power to the front and/or rear tires for 4WD drive and AWD vehicles.
The differential is a system of gears that allows different drive wheels (the wheels to which power is delivered from the engine) on the same axle to rotate at different speeds, such as when the car is turning.
Both the transfer case and the differentials require fluid to lubricate the internal components and to pull heat away from the working components. This fluid can be degraded by heat and/or external contamination (dirt and moisture). We recommend having your transfer case and differentials inspected and the fluids changed every 30,000 miles or if you experience the below symptoms.