Hey just wanted to follow back up on this thread. We replaced the thermostat housing, since that seems to be a very common problem. It looked like it had been done not too long ago (he's had the truck about a year so it would have been longer than that).
I called the local ford dealer and they wanted about $180 for all the parts. After reading about all the problems with Dorman aftermarket parts, we decided to go for the
aluminum replacement unit from Austek but got it from ebay seller "bfoot58" ("
Armadillo Auto parts") for $81 shipped.
Its a really nice unit and includes both sensors, silicone water pump bypass hose, thermostat, and all the necessary mounting screws. A complete drop-in unit. Tough to beat that deal.
In order to install it, I pulled both sensors and the upper t-stat housing, then slid it in under the intake and screwed it down , then plugged the sensors into their harnesses and installed them into the holes, then finally installed the upper t-stat housing. Very simple and it fit great.
The good news is that it doesn't leak any more, so that must have been the source of the problem, even though I could not see ANYTHING wrong with the unit we removed.
One other thing I noticed is that someone previously replaced one of the sensor harnesses so that both sensors are the "gray" body units (originally one is gray and one is brown and the connectors are different). For some bizarre reason the PO (or his mechanic) clipped off the old connector to the brown type sensor and replaced it with another gray type sensor and connector. I guess they must both be the same?
Any way, thanks for the help on this one. 203,000 miles and back on the road running strong, hopefully for another 100,000 miles.