The Yukon Trac Loc spring is intended for Ford 8.8" differential housings with 31-spline axles and supplies the spring components needed to install a Trac-Lok limited-slip carrier. Yukon lists this kit specifically for Ford 8.8" 31-spline applications and provides the parts necessary to enable or service Trac-Lok style limited-slip operation in those axles.
Yukon Gear Trao Loc Spring For Ford 8.8in / 31 Spline