"Letter of Reprimand" damaging

I was pending a divorce with my ex-wife when allegations of assault and trespass were made. The Assault was dismissed and I'm on a 12 month SOC with the trespass. My agency had me on admin leave for 6 months, and then brough me back to work full duty. I received a "letter of reprimand" as the "disciplinary action", as my agencies administrator apparently found (by nothing more than my wifes allegations), that I had been involved in "misconduct" while off-duty.

My issue is that the "letter of reprimand" is outright abusive and totally innaccurate. It says that I am "violent and abusive", and makes other claims that are just attrocious. I have no history whatsoever of any negative interactions with the law, and no history of any disciplinary action with my current or former agencies. The letter is a page and a half of accusations, opinions and belittling.

It will remain in my employment file for 36 months, killing any opportunity for promotions or advancement with this or any other agency.

I am trying to determine if hiring a local employment lawyer to write a letter to my agency with my concerns would likely be of any help or might convince my agency to pull the letter from my file. My union won't take action on it because it is "just a letter".

Thanks for any advice or words of wisdom..