Hello Rob, and welcome to JustAnswer.com!
The power sliding doors on these can be a real pain to diagnose, and access to a ford diagnostic tool (such as the NGS or WDS) is necessary to do so, so this is most likely going to be a dealership job...
BUT there are a few common problems you can attempt to fix yourself.
First, at the sliding door and at the body where it latches to you'll see an assembly with 'fingers' sticking out, and a pad with metal plates the fingers press up against when the door is close. Try cleaning all of these contacts with a bit of battery terminal cleaner and steel wool. Sometimes a bit of dirt or corrosion can build up, and a solid electrical connection isn't made once the door is closed. (#9 in the illustration below)
If this doesn't do the trick you can try adjusting the striker. This is the small bar the door latch actually grabs on to to hold it closed. You can loosen the bolts, pull outward on the striker, and retighten the bolts. This will only allow for maybe 1/32 to 1/16th of an inch of adjustment, but sometimes this does the trick.
Hopefully this will take care of it for you. Good luck!