I have an 05 Ford Freestar 3.9L that was running and driving good when I parked it last spring. I started it and let it run once a month to keep the battery up. We decided to sell it since we don't need it and today I went out and after checking all the fluids I started it to let it warm up. When I put it in reverse it didn't do anything but seemed to remain in park. It won't go into any gear at all. The engine rpms rev in all gears as though it's in neutral or park. Any ideas? Was suggested it may be the trans position sensor. HELP!