Just pretend everything you post or is being said directly to a reporter. If you wouldn't say it to a reporter, you shouldn't post it. It seems like overkill to turn the investigation over, but you want an impartial 3rd party to investigate and/or clear the officer. You don't make public jokes like that. Public servants are never totally removed from our titles. For better or for worse that means we can't or post things like this publicly. If you chose this line of work you will be held to higher often unrealistic standards both at work and off duty. If that isn't ok with you, GTFO.
edit- I just had someone email me to point out that other professions get fired for facebook posts etc too... I agree... but there are certain professions that the news will tack onto your shame story every time. You rarely see "Local plumber arrested" or "Area tool and die man accused of threats"..... but replace those jobs with Police Officer, Firefighter, EMT, Teacher, Judge, Physician.... and you have a story. These are all positions that come with a higher expectation of socially responsible behavior.