The Grateful Dead's "Truckin'" (1970):
In the 1920s and 1930s, black jazz musicians used the verb "to truck" in the Robert Crumb sense of "Keep On Truckin'." You can see an example of it in Disney's 1941 "Dumbo," where one of the crows says "And they tell me that a man made a vegetable truck," and all the others crows assume the Keep on Truckin' position. link
Here's the scene from "Dumbo;" the line occurs at around 58 sec:
The crows do a lot of stereotypical strut and swagger, and I wonder if R Crumb was influenced by this in creating his iconic cartoon? Terry Zwigoff's documentary "Crumb" recorded how the young Crumb brothers were obsessed with at least one Disney movie, "Treasure Island" (1950), and Crumb probably saw Dumbo too as a kid (what boomer didn't?). Also, Crumb showed an early fascination with -- ahem -- African American caricature.