F-250 Ford-PU 4x4 - shifting to 1st/2nd gear on long down grade chews gears up immediately upon shifting; automatic 1999; also - 4x4 won't engage properly; anyone had this problem? what did you do; can you refer me to someone who knows what will cure this; I've had it worked on 2 times and still does the gear-chew and I will be pulling 33 foot trailer down mountain roads soon.