I suspect that you could talk to almost any old-timer at any local district state Dept. of Transportation to find out the typical kinds of things that a driver would do.
I suspect he was driving a truck to haul rocks, gravel, sand, caliche, etc., for the building and repair of roads. Since he had a history of teaming, he may well have been using horse- or mule-drawn fresnoes to move dirt. Those were still in use in the 1930's.
I don't know if you could find out the official word or not, but you could try to get in touch with the NM State Dept of Transportation in Santa Fe.
Elvis E. Fleming