I'm betting it's the actuator, especially if it's an 01. They fail eventually one by one. There is a resistor inside the motor that is encased in the actuator. You can take the actuator out, remove the motor, bypass the resistor and put it all back together. 100's of guys, including myself, have done this "foil mod" and it works like a charm. If you are adventurous and want this 'free' fix, let me know and I will post a link.
2001 F350 CC SWB 6spd....currently getting a complete 'makeover'. Stay tuned.