I had the same problem. It was the brake switch. I had to slam on the brake and pull the gear shifter down at the same time. It actually saved my truck from getting stolen. The theif broke the door handle, turned over the ignition and snapped the gear shifter off the coloumn and gave up!
One way to know if its the switch is if it is locked up, turn the key into the first position, (no longer need to step on the brake to pull it out of gear) put the truck in neutral and then start the truck and put it in drive.