re:keep in mind that this 'truck' is not the same as the mode of transportation, which has rather a different etymology.

It does? i thought the two meaning divirged from "truck farms" (truck meaning trade farms.. in the 1950, NJ was still so rural, (even the north end) that its prime industry was truck farmers... i.e., farmers who traded there own goods.. and it was in the 1950's i learned truck can = trade goods/trading.)
(See WO'N, i know NJ really used to be a garden state!)
and from there, the meaning of truck (trading)transfered to the vehicles (flatbed trucks) that were use to move the goods. (and other type of trucks for transportation, (hand trucks, and general vehicles)

so if truck (vehicle) is not a transfered meaning from truck (trade goods/trading) where is it from?