Legal and professional advise I highly agree with.
To have a civil employee picketing a religious organization could be a black eye on the city/county/state, as these days civil employees are required to be squeeky clean in the public eye, and picketing is very public. However, this doesn't mean you have to quit... just stay in the background.
After getting legal advice, contacting the ACLU first, I suggest contacting the local authorities regarding getting permits for public gathering or picketing, whatever it's called in your area. Once those are in place, contact the local police authority and discuss it with them in person, and bring literature! Maybe Mikronboy could provide you with written copy of his anti-witnessing on Ebay regarding the Library Card. Mikronboy is not a JWD handle, but pm me and I'll tell you who he is. Online sites with listings of Cult and High Control groups is a good place to look for info, too.
Now, a few picket signs made to stick into the ground like political "vote for" signs (not posted to a light pole, which is illegal) might be a good temporary anti-witness. Placed in public property right of way out of the street of course! Watch it, and when a witness goes to remove it, reprimand them for violating your freedom of speech - all on video, which is being filmed in plain sight. Video vs audio, does not require the knowledge of the persons being taped, and thus is legal in court. Don't fight the butthead if he forcefully takes it. Just put others out when the butthead leaves. You've got the evidence for violation of first amendment on tape, and you are not being violent!
Good luck and gawd bless!
Brenda