just get a wiring diagram and start narrowing things down with a volt meter or test light. sounds like you have a ground somewhere that could be bad either that or somehow both bulbs burned out. because when you switch to high beams both low and high beams are not on, just highs
Because the headlights draw higher amps, there's usually a relay just for them. If neither bulb works, start looking for a high beam relay. It'll be in the relay main power distribution box, usually on a wheel well liner. Fords seem to like the driver's side.