I have a 2003 silverado 2500HD with 6.0L gas. so i parked in my driveway to hook up a trailer with 4 wheeler on it and when i wentto start the truck to leave it wouldn't start and the gear selecter kept going back and forth and it stoped on low 2 and it won't come out of it to start, even though the truck is in park . Does anyone know what might cause this. Any help would be great thanx.