F-250 Ford-PU 4x4 - Shift to 1st/2nd gears chew up-SAFETY HAZARD

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.
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