I have stood at my local Ford dealer and listened to them have the exact same conversation with a customer complaining about this problem. By the way, they gave the guy the exact same answer.
I have same the problem with one of the headlights on my '97 Ranger -- for whatever reason, the other headlamp is fine (?). I have tried taking the bulb out, blowing the housing out with compressed air, etc. Nothing works. In fact, this truck is currently being stored and I can walk out into my garage right now and see the moisture in the housing -- even though the vehicle hasn't been out of my garage in over two months! There's no discoloration of my lens though -- it's just that you can see the moisture in there.
One of these days I'm just going to break down and replace the affected housing because it kind of bugs me to look at the moisture inside the light.