I want to depict an M35A2 for the US National Guard, but how do I paint and mark it? I have found no inofrmation on how National Guard vehicles are organized and marked.

Depends on the National Guard unit. Some state Guard units are arranged into combat formations like the Regular Army. They are marked like any divisional style units. Some units are non-divisional and are marked as such. Some units belong directly to the state HQ command.

Remember, the National Guard isn't one large stand alone organization. It is more like 50 something various autonomous state/territory commands with various sub-units. Some of these sub-units even cross state lines, for example, the 42nd Infantry Division consists of National Guard units from NY, NJ, and various New England states.