Just after World War II, Willys Jeeps and Dodge Power Wagons were first to sell 4WD trucks to consumers. GM, Ford, and even Studebaker saw the demand for trucks with a transfer case, but before they could offer their own 4WD systems cost-effectively they outsourced it to the aftermarket.
The Northwestern Auto Parts Company (NAPCO) was a small upfitter that got organized out of Minneapolis in 1918. The Second World War gave them a big boost as they were contracted to provide four-wheel-drive systems, transmissions, winches, dump truck bodies, and other such upgrades.
But their big commercial endeavor for the consumer market was the Powr-Pak 4x4 conversion, which was sold to customers at GM dealers and NAPCO shops for $995 in the 1940s. That's seriously expensive option considering pickup trucks were listing at something like $1,500 back then.
The Powr-Pak was simple and easy to install, ergo actually quite clever. Using 85% GM parts, a two-wheel-drive GM truck could be converted to 4WD by drilling four holes into the frame and modifying the front axles to accept constant velocity joints which allowed the truck to steer. The transfer case was hooked up to the transmission via a shaft and voilà; 4WD GM pickup truck.
Front axle differentials, brake drums, wheels, drivelines, backing plates, springs, shock absorbers, and universal joints were all swapped in the process, though apparently it only took mechanics about three hours to perform.
According to Hemmings; "prior to 1955, only one ton and three-quarter ton GM light trucks could be converted. GM's 1947-1955 half-ton trucks used a torque-tube style driveshaft that wasn't easily adapted to a transfer case. Once the torque tube was eliminated in the second-series 1955 trucks, it was possible to use NAPCO conversion kits on half-tons."
GMC offered the half-done kit at dealerships in 1956,with Chevrolet getting on board in 1957. In 1960, GM redesigned their pickup trucks and started offering their in-house built 4WD system.
NAPCO hung on for awhile creating kits for very large trucks, but were bought out by a major axle company you may have heard of called Dana "sometime in the 1960's" according to the folks of the NAPCO Owners Club.
In the 1980s, NAPCO became NAPCO International and continues to this day building, servicing, and upgrading military vehicles.
Image: Chevrolet 3600 Stepside Pickup (3E-3604) 1957