Originally Posted by Alex_M

If it will only run when the park brake is engaged then your rear end let go. Most likely the spider gears in the rear diff, possibly the ring and pinion, maybe even an axle. My money is on the spider gears. With the park brake engaged it is allowing you to drive on the front end.

This would be a 3 or 4 hour job to fix simply by removing the rear 3rd member and dropping a new one in out of a junk yard truck.