This can be caused by several problems, this truck has an interlock system on the shifter so you have to press the brake to start and shift it. Sometimes dirt gets in the shifter solenoid and make s it stick, or it just starts to go bad, also the key lock system in the steering column has a set of tumblers that have to be in sync for it to turn back and forth, these can fail with constant use or dirt or both. This problem because of the mileage on the truck can be very hard to pin down which component is bad, each one will have to be tested out of the system to see if it is bad or not due to the complex nature of this system you would probably be money ahead just letting a shop find and fix the problem. Thanks for using 2CarPros