It was popularized in the late sixties by cartoonist Robert Crumb. His "Keep on Truckin'" cartoon in Zap comics (1968), featuring a guy leaning way back with his index finger up and his foot thrust forward. It was a popular subject of poster art in the late sixties.
The Grateful Dead recorded a song called Truckin' in 1971. By then, the term was pretty much over, so the Dead had nothing to do with it. Eddie Kendricks recorded a song called "Keep On Truckin'" in 1973. Hot Tuna, a Jefferson Airplane spinoff, also recorded a (different) song under the same name. "Truckin'" was often a euphemism for a similar word with which it rhymes.