This is discussed at length in the Excursion forum at ford-trucks.com.
My rear lift gate lock has stopped working twice now in my 14 months of ownership. It didn't work when I bought it and I just had to fix it again a couple days ago. All it needs is a good clean and lube.
Access the locking mechanism by removing the inside lower panel on the lift gate. Here's what I did - using compressed air, I blew all the dust out of the lower lift gate area. I then sprayed the mechanism down very good with carb cleaner. Sprayed some more while working the key back and forth. I then sprayed silicone on the mechanism again while working the key. At this point the mechanism started moving it's full motion and lock and unlock was working again. With a flash light, if you look closely, you can see how easy it is for this mechanism to bind up if not perfectly clean and lubricated. I used a q-tip and some synthetic wheel bearing grease and worked some grease in the best I could (not sure if this will help or hurt; time will tell). Obviously it's a lousy design. The lift gate lock on my '91 LandCruiser has always worked perfectly without me every touching it. Part of the problem appears how easy it is for dust to get into this area. Mine was very dusty after spending a lot of time on dirt road this spring. In an attempt to limit the dust intrusion, I sealed up the 3 holes towards the center on the bottom of the lift gate. That still left holes on each end for drainage. I don't think it will make a big difference, but anything to help can't hurt.