This is called freedom of speech, I would go above her and tell them you will get a lawyer if need to [just a bluff]. They can not do this . I would ask them to see the rule that you can not have it on your truck.
Allso when I had a very similar problem in school I put it in the papper BOY it got fixed fast then.

good luck I say stand your ground.

Whish you lived close. Where did you live?