I have a 2008 Ford e450 and this same thing happened to me a couple of months ago. Unknowingly, I had been under the dash connecting a wire for my rear camera and accidentally pulled off the wires to the switch that controls the disengagement of the gear lever. This switch has nothing to do with the stop light switch, but is a separate switch which has a push button which is depressed when the brake pedal is in the upper position. When you step on the foot brake, the button is released, enabling the gear shift lever to be taken out of park. This feature is in almost all vehicles to ensure you have your foot on the brake when putting the car in gear. Check that the wire bundle going to the switch has not pulled away. The switch is directly above the brake pedal.