Awwwwwwe, I was going to say that.

 

Put the key on engine off, parking brake on or a block behind a tire in case your on a decline. Put the car in reverse, and listen to each sensor. You should hear each one making a clicking sound. When you put your hand close to it the clicking should change. If there is no clicking from one that's the bad one. I know everyone says to only buy genuine ford parts, and that is the right thing to do usually, but in this case if its a car or truck that you have to take the entire bumper cover off of to change the sensors, I would just buy 4 on ebay for $35 and replace them all.

 

No need to remove the bumper to replace them on the Expedition, you release them by spreading the tabs on the inside of the bumper, then pushing the sensors in and remove them from the inside of the bumper. I've done this myself on mine, was a 10 second job once you figured out where the tabs where.

Not sure on the Navigator though, it has a different radar thing in the middle which is quite expensive to replace if it goes bad.