You really need to settle down, it's an expired tag not a DUI. 99% of all states it is a civil infraction not a criminal violation so the very most that can EVER come of it is a nominal fine. You want legal advice, you have already been given it by a few people. If that's not good enough for you, hire a lawyer to do exactly what you've already been told for a lot more money.
I'll tell you this, most people, myself included, don't bother checking a vehicle or equipment for proper tags and insurance paperwork and that's your fault and why you got a ticket. Now here's your "free" legal advice...I was a cop in Phoenix and one of my specialties was running traffic. I most likely would have given you a warning, since I would have known through NCIC that the trailer was not yours but who knows...approach determines response. I know it's not your trailer but if your buddy was a true friend, he would have already gotten the trailer registered after you told him about the citation.
So, before your court date, call the court and ask them if you still have to attend the hearing if you provide proof that the trailer is registered. If you do, go to court and dress respectably. Provide the judge with the proof that the trailer is registered and beg the court for mercy and say that you were at fault for not acting like a responsible motorist by checking that all documents and equipment were in order.
Take a Xanax, you'll be fine.