Not sure which state you are in, but in Missouri, as most states, you can take him to small claims court. The limit in MO is $3,000.00. As far as warranty, I believe Federal law states that any item must last a "reasonable amount of time", and warranty time frames are secondary to this principle. Regardless, unless specified, the tranny problem should have been disclosed before purchase and I'd say you have a case.
Before you do this, however, take the vehicle to a NON-transmission repair shop, and get a second opinion. $2,900 is a lot for a tranny, and when you go to a tranny shop (in my experience), the only outcome is that you need a tranny. The truck may not even need one.
