Tranny feels like it's downshifting when accelerating from a stop.
 

So my wife gives me a call and says her 2001 F250 Superduty "won't go". I tell her to pull into a parking area and wait for me. With the auto tranny in OD, it feels like it's downshifting as soon as it starts picking up speed from a stop. I can't get past 20 mph. I hit the OD button on the gear selector and it still does it. When I put it in 2nd gear on the selector it will go down the road with no apparent problems. I checked the fluid level which is fine and it appears to be clear. Prior to this episode, no problems. Any ideas? Any tips would be greatly appreciated.