I've yet to find anything within reason to stop road ragers from having a conniption.
Maybe get one of those car LED displays, a rear mounted light bar, or rear mounted Hellas. Someone starts raging, send them a message or just leave the lights/horn on blast. Not safe or legal but the message gets sent.
Really though. Maybe find a nice bumper sticker or tag relating to PTSD. Might get some folks to lay off and it's not inaccurate due to past victimization. Military tags might also work, but putting those up walks a fine line...