Just a guess in general for automatics, but stuck torque converter lockup?
How long was she driving at highway speeds before it started doing this?
A couple of things to try... First, get the vehicle to where it's safe, nothing in front of it to hit. Then, start the engine in neutral, not park, then rev the engine a little, then shift from neutral to drive. Revving the engine is to keep it spinning. It should launch like popping a clutch if it's a torque converter lockup sticking.
I had this happen on my '78 Cordoba, first year they introduced the lockup. It was entirely hydraulic, and as it ablated, the material stuck in the control valve and made it stick engaged, which of course made it ablate more, leaving me with one helluva problem.
I'm not a Ford expert, so your mileage may vary.