Ben,
Thank you so much for putting this how-to out there on the net.  My 2004 Explorer Sport Trac had the lower housing split in the typical 2'oclock rear position and I just replaced it with help from you!
 
 http://bengrosser.com/howto/fordthermostat/
  
  I thought you might like to know that some 4.0L have the lower housing with only one sensor (the temperature sensor).  The housing has what looks like two places for sensors, but only one "knocked out". So, the part numbers and Ford engineering numbers are different.  I have listed them below so that people won't accidentally order the wrong housing and be stuck.  My local Ford dealer here in Atlanta, Frank Jackson Sandy Springs Ford, had the lower housing w/one sensor hole for $57.69 + tax.  $61.73 total.  I got the sensor, lower seal, thermostat & t-stat seal at the local AutoZone for a total of about $30.  I got the bypass hose from NAPA for $6, but ended up leaving mine alone and returning it.  The dealer actually gave me a free sensor o-ring, even though I already had one on my new temp. sensor.  They had a whole kit of them available.
   
   Here are the part numbers/Ford engineering numbers I mentioned:
    
    Lower Thermostat Housing (2004 4.0L Sport Trac, 2WD)
     
     1 Sensor (pop-in, not threaded):
     Engineering Number (on bottom of housing) = 2L2E-9K478
     Ford Number = 2L2Z-8592BA
     Motorcraft Cross Reference # = RH148
      
     2 Sensors (pop-in, not threaded):
     Engineering Number (on bottom of housing) = 2L5E-9K478
     Ford Number = 2L5Z-8592BB
     Motorcraft Cross Reference # = RH144
       
     Just thought you might like to use/share this info. if need be!
        
     Thanks again,
     Chris