The most likely cause, IMO is the ring and pinion malfunction as described earlier. My guess is that this is a 489 case 8.750 rear. I bet dollars to doughnuts that the crush sleeve was put under stress and crushed too much resulting in incorrect pinion angle and causing the lockdown. It's not too uncommon. That's the weak link in the 489 case. I would get a crush sleeve eliminator anyway (assuming again it's a 489) and replace the crush sleeve. It will make the rear much stronger and you'll never have to worry about it again.
