I have a 2001 Explorer Sport Trac with the 4.0L engine. The upper radiator hose is attached to a synthetic housing. Is this the thermostat location? I was sold a rubber O ring to seal the thermostat, but this housing has a flat gasket that appears to be leaking. Do I need a different gasket for this housing? Thanks for any info.

Your vehicle uses the "O" ring and not a gasket. Look closely to be sure the housing is not warped or cracked while you have it off. It is located where you described.

Yes that is it, mine was leaking as well. I had to replace the outlet as well as the seal and thermostat.