I just replaced...well fixed mine for now. The local auto place didn't have the part so I took the actuator apart turned the gears 180 degrees, shaved off one of the tooths of the gear that fits into the actuator and it works. It was so easy to get to that I figured any future problems will be easy enough.
First pry the cover off from around the radio.
http://www.expeditionforum.com/attachment.php?attachmentid=6967&stc=1&d=1376361146
Click on the link and the picture shows the 6 fasteners that will come out with a little prying especially the closer you pry next to them.
Now use a 7 or 8mm to remove the Nav radio.
http://www.expeditionforum.com/attachment.php?attachmentid=6968&stc=1&d=1376361696
The actuator is right behind the radio. I put the key to accessories and put my hand on the actuator to feel it clicking so that I new for sure that's what it was.
http://www.expeditionforum.com/attachment.php?attachmentid=6969&stc=1&d=1376361766
This picture shows the front screw (with a little arrow to the back screw) and the wiring attachment to the actuator. The screws are an 8mm if I remember correctly (all you need for this job was a 7mm and an 8mm and some string to tie the wrench to your belt loop...if the wrench falls its VERY hard to find and get back up).
The back screw can be loosened with the box end of the wrench with the use of just finger tips...just remember, tie a little string to it. I used duck decoy line since that was all I had.
http://www.expeditionforum.com/attachment.php?attachmentid=6970&stc=1&d=1376361766
This is just another picture of the two screws that hold the actuator in place.
Notice how the gears that go into the blend door can only go in one way.
At this point you can get a new one and reverse the order to get it in place or if you want to fix it for now and not wait on the new part like I did continue reading...
Once the actuator is out you should be able to see all the little snap pieces around the sides and how to get this thing apart. Inside is a little motor, a bunch of gears and a printed circuit board. The biggest gear was the one with the busted tooth.
http://www.expeditionforum.com/attachment.php?attachmentid=6971&stc=1&d=1376361766
The arrow shows the broken tooth and the circle is the side that I put back in. So basically I turned the gear 180 degrees. Close it back up and turn it upside down. You'll also have to shave off one of the teeth from the fitting that actually goes into the blend door since there is originally a gap so it can fit into place only one way.