What you need to do is the following:
1. Call the General Manager of the Nissan dealership that is giving you the troubles and explain that if the matter is not solved you will make a formal complaint to Nissan that their dealership is giving you the run around.
2. Contact Nissan and explain thet you bought THEIR product and they should stand behind it and if this matter is not fixed in a timely fashion you will start calling the local TV stations and park your car in front of a Nissan dealership every weekend with a sign that says "Don't buy a Nissan they won't back their product"
3. You need to make a complaint with the better business now(gotta get the problem documented.
4. Take the diagnosis form the transmission shops that you have and make a complaint to whomever licenses dealerships in your state.
I promise you that if you do these things you will see some response from them.
One thing as long as I have been in the car business that a dealer does not want is BAD PRESS or a customer standing outside his dealership on a weekend saying "Don't buy these cars"
Finally check your states lemon laws
http://autopedia.com/html/HotLinks_Lemon2.html