A movable load support assembly for mounting storage units such as tool boxes onto a truck bed is provided. Guide rails are mounted on opposing side walls of the truck bed. The tool box is mounted on a pair of support members each of which includes a roller assembly to allow the support members and storage unit to freely move along the guide rails. A locking mechanism is provided to securely position the tool box at a desired location along the guide rails. Means are provided to allow the assembly to be used with trucks having nonparallel side walls.