Truck isn't driven much. Just turned 13K. I went for a ride today and on my way home I noticed the engine revving slightly. The dash indicator showed a "4" in the gear window. The "check engine" light also came on. It would not shift out of 4th. I stopped on the side of the road and restarted the engine. It shifted okay for about a mile then the same thing happened. Any ideas as to what is causing this? I really love the truck. This is very disappointing for such a "new" vehicle. Thanks for any input.