These 4 pin limited slip Trac Locs are for Ford 9" 28 or 31 spline applications. These are new, not rebuilt or remanufactured. Our Trac Loc 9" case is stronger than the OEM unit. It is a steel case with extra support material in the critical areas. Our Trac Loc fits all 9" rearends and is for heavy duty applications. This product does require the use of friction modifiers.