One problem related to transmission not go into gear has been reported for the 2012 Jeep Wrangler. The most recently reported issues are listed below.
The transmission became difficult to engage 1st gear. Shifting into all others was very notchy. Finally, in traffic, it became impossible to engage 1st gear at all. The vehicle had to be started in 2nd gear. The vehicle had limped in to the dealer. Upon inspection it was found that the 1st gear synchro had cracked and failed. This can create a hazardous situation in busy traffic. This transmission has been reported by various user forums as faulty and prone to failure, including popping out of gear and difficult shifting. Popping out of gear in a busy intersection could be dangerous, especially if you are unable to engage another gear. The dealer is repairing the broken synchro.