I have an 88 f250 7.3 idi 4x4. Today I was driving and the truck seemed to have went in to neutral. When I got it pulled over I saw that the drive shaft was turning while in reverse or drive. When the tow truck dropped it off, I put it in park but it tried to roll down my hill. Had to chock the tires. There was no noticeable noise from the rear-end when this happened. Any ideas?