Let's talk about NAPCO trucks. Northwestern Auto Parts Company, picture of original building to left, started up their four-wheel drive conversion business in 1918. NAPCO really didn't get their business going good until after World War II. The government contracted with businesses like NAPCO to make their war vehicles more powerful off-road. After the war, NAPCO turned their energy toward the private market and made conversion kits for trucks so factories could turn a regular truck into a 4x4.
It didn't cost much and it was more powerful than any other method. An interesting little site about these conversion kits is History of NAPCO trucks