Why would a Ford 2000 Expedition just lose power and shut off as you are driving?

More than likely the loss of fuel or air. What year is the vehicle.

It could be the Crank Position Sensor on the front of the motor. easy change, 2 bolts and a electrical connector.

this truck is a2000

Above answer is good, however I run a wrecker company and most of the time when one of the 7.3's stopped, they were low on oil. Simple, but check the oil first. Fuel pump can also cause it, plus the above.