Your vehicle may have two differentials if it is four wheel drive, but the service is the same regardless. The fluid is removed from the unit by removing the unit cover or a drain plug if equipped. A drain plug may be added by us if there is a suitable location for one, this will facilitate future services. The fluid contained in the differential(s) is a higher viscosity (thicker), but like any oil, has additives added to keep seals soft, detergents to clean, as well as other ingredients. These additives break down and dissipate as they absorb heat, by replacing the fluid, these additives are replenished. The recommended service intervals are: 24,000 miles-city and 36,000 miles-highway.