A movable subframe or slider for semi-trailers includes a pair of spaced-apart, parallel and longitudinally extending main members. The main members are interconnected by a pair of longitudinally spaced, parallel and transversely extending horizontal cross members, a horizontally-disposed cross-brace structure, and a pair of longitudinally-spaced, parallel and vertically-disposed cross-brace structures. One or more axle/suspension systems are suspended from hangers which are mounted on and depend from the slider structure, and a retractable pin mechanism enables selective positioning of the slider relative to the trailer body for optimum load distribution and trailer versatility during vehicle operation. The integral slider structure supports the one or more axle/suspension systems, so that concentrated loads imposed on the slider via the axle/suspension systems during operation of the vehicle generally are dissipated throughout the entire slider structure.