Here are some photos of 1st & 3rd Mar Div trucks in Vietnam. The deuces and 5 ton are 1st Mar Div and are either in Danang or on convoy to/from Danang. The 1st used a yellow shield shaped device and was often seen on both driver and passenger door, front center of hood and rear tailgate. It often had yellow unit lettering/numbers to either side or sometimes had red lettering within the device itself. Quite a few 1st Mar Div trucks had "F L C" in red within the yellow shield. This stood for Force Logistics Command, which was primarily based in Danang.
I'm not sure where in Vietnam the 3rd Mar Div M37 was photo'd. It shows, be it somewhat blurry, the red shield device with a yellow caltrop (3 pointed thingey) within it. Again, it often had unit info to either side of device.
However, be advised that not all trucks were marked with unit ID, and even when marked, they showed quite a bit of latitude in both the method and manner.