The differential allows one axle to turn at a different speed than the other while distributing power to the wheels. The inside wheel spins slower than the outside wheel when turning. Since the differential has gears and bearings that are lubricated by fluid, a differential fluid replacement is needed maintain the proper lubrication of the gears and bearings.
Most maufacturers recommend that the differential fluid be changed every 30,000 miles. If the vehicle is used to tow or carry heavy loads, then the fluid may have to be changed more frequently. Refer to your manufacturer's specification on when to do a differential fluid replacement.
To perfrom a differential fluid replacement, the vehicle needs to be driven in order the get the differential fluid warm. Then the fluid is drained and replaced with fresh fluid.
Wether your vehicle is used to carry heacy loads, or just passengers, you would be best advised to follow the factory recommended vehicle maintenance schedule for how frequently an differential fluid replacement is required. This factory recommendation must be adhered by to maintain the validity of your new car warranty and extended auto warranty, if applied. If mechanical breakdown occurs due to the lack of your car's maintenance of the differential fluid, the warranty would not cover the car repair.