Nice layout above!
It’s true that the US Army does not have any field guns in its inventory at the present. The last field gun was the M107 175mm self-propelled gun. This system was replaced in its role providing long-range fires by field artillery rockets by the 1980’s. However, the M107 was widely exported to US allies. In Twilight: 2000, this system might be found with German, Iranian, Greek, South Korean, and British formations, among others.
http://en.wikipedia.org/wiki/M107_self-propelled_gun