Using a series of gears, bearings and clutches – your car’s differential is in motion any time your vehicle is moving. Some four wheel drive owners believe the differential service is not important if they don’t use 4×4. The truth is, the gears in a differential mesh with each other, whether the truck is in 2WD or 4WD. This meshing, along with the clutch plate friction any time you turn, will wear out the differential fluid in time.