A retractable under-guard, for example for dual-purpose trucks that travel on-road and off-road, provides impact protection to other vehicles colliding with the truck, but is raised for off-road travel to avoid obstacles. The under-guard connects by a frame wrap to horizontal frame beam(s) of many different trucks and does not hang down in its retracted position to an extent that it interferes with off-road travel. For entering the road, the guard frame pivots downward to the desired position at or near the outer perimeter of the truck, and typically about half way between the truck frame and the ground in the space into which a lower vehicle would enter in a collision. Hook and pin systems, and/or impact plates, maintain the under-guard in proper position upon an impact. The under-guard system may automatically deploy above a predetermined truck speed, to ensure deployment when the truck re-enters the roads.