If you feel like you need legal advice you should talk to an attorney before you say anything to your employer. FR isn’t the place to get legal advice.
Academic Freedom is what I would cite and I would also ask if anyone else posts on facebook, the DU or any other website as an employee and bring up the possibility of discrimination being practiced by the university by persecuting you. Most employers could claim, my computer my rules, yet as a professor you are entitled to academic freedom. Does free republic help you prepare for classes with ideas or discussion? That being said I would pull an attorney into the meeting just to let them know you are serious if they want to go there.