I've read several posts that indicate the auto door lock operation is a common problem with Excursions.

What gives?

I've had a few times lately where one of them (not necessarily the same one) won't lock. It's making me paranoid !! My wife thinks I'm crazy for locking the truck and then going around the truck checking all of the doors !!!!!!!!!

 

Door lock accuators have been a problem. Many have replaced them with an aftermarket unit. Others just replace wiht a Ford unit. Don't know how old yours is, but if it's 2-3 years old, it's likely got one or more going out.