Abstract: A boom for lifting and supporting a vehicle to be towed is pivotally supported at one end thereof on the bed of the towing truck. A hydraulic cylinder is pivotally supported on the truck and has its ram pivotally attached to the boom. Also pivotally attached to the boom is a bracing post which is slidably fitted into a support member, which is pivotally mounted on the truck. A ratcheting lock mechanism is mounted on the support member for locking the bracing post relative thereto when the boom has been driven to the towing position. The ratcheting mechanism permits upward travel of the boom but prevents downward travel from any one of several locking positions except upon manual release thereof.