Background
Excerpted from Wikipedia:
The M54 5-ton 6×6 truck (G744) was the basic cargo model of the M39 Series truck. It was designed to transport a 10,000 lb., 14-foot long cargo load off-road in all weather. In on-road service, the load weight was doubled. The M54 was the primary heavy cargo truck of the U.S. Army and U.S. Marine forces during the Vietnam War and was also used by the U.S. Navy, U.S. Air Force, and ARVN forces.
The M54 truck was used as the base vehicle for Gun Trucks during the Vietnam War. Field-applied armor added protection to the Gun Trucks, and armament consisted of various combinations of weapons including M60s, .50-calibre machine guns, XM 134 miniguns, and anti-aircraft weapons such as the quad mount .50 cal. machine guns. Early 2 ½ ton trucks proved inadequate leading to their replacement by M54 five-ton cargo trucks.