As to the Chaffee
Keep in mind that Norway operated them until 1993 in the real world, Uruguay still operates them today and Chile operated them until 1999. And the US, with a lot of its factories shut down by early 1998, would have been raiding every collector, armory, museum and arsenal they could find for tanks and other armored vehicles. So the idea of them being able to put a few back into service is not that farfetched.
And I will take an old tank with a 75mm gun on it any day over an armored car with a few machine guns or a gun truck.
Keep in mind places like
http://www.site.ww2mv.com/ for places the military might find old tanks they can use in a desperate situation like the US is in by late 2000 of the Twilight War.