I've seen them at least twice on /r/whatisthisthing, a good couple dozen times on the road, and these days, even in press photos: GMC trucks with custom square boxes on the back, painted dark blue, with US Government "E" plates. These courier escorts, "unmarked" but about as subtle as a Crown Vic with a bull bar, are perhaps the most conspicuous part of an obscure office of a secretive agency. One that seems chronically underfunded but carries out a remarkable task: shipping nuclear weapons.
The first nuclear weapon ever constructed, the Trinity Device, was transported over the road from Los Alamos to the north end of the White Sands Missile Range, near San Antonio, New Mexico. It was shipped disassembled, with the non-nuclear components strapped down in a box truck and the nuclear pit nestled in the back seat of a sedan. Army soldiers, of the Manhattan Engineering District, accompanied it for security. This was a singular operation, and the logistics were necessarily improvised.
The end of the Second World War brought a brief reprieve in the nuclear weapons program, but only a brief one. By the 1950s, an arms race was underway. The civilian components of the Manhattan Project, reorganized as the Atomic Energy Commission, put manufacturing of nuclear arms into full swing. Most nuclear weapons of the late '40s, gravity bombs built for the Strategic Air Command, were assembled at former Manhattan Project laboratories. They were then "put away" at one of the three original nuclear weapons stockpiles: Manzano Base, Albuquerque; Killeen Base, Fort Hood; and and Clarksville Base, Fort Campbell [1].