One problem related to transmission not engage has been reported for the 2009 Jeep Wrangler. The most recently reported issues are listed below.
A handful of instances over the past month, my 2009 Jeep Wrangler violently and unexpectedly pops out of first gear upon acceleration. This could result in a rear end collision, potential accident while crossing traffic lanes or upon acceleration onto a highway like the sawmill parkway where you enter the highway from a dead stop. In addition to the potential catastrophic results due to an accident, the shifter when it violently disengages can result in a damage to the drivers hand or fingers. My fingers to a significant hit the first time I experienced the problem. Through a search on the internet, I see that this has happened to other Wrangler owners.