..or simply get any odb2 reader...rented,bought, or borrowed..all will read from engine temp sensor. One's that can read the specific can-bus for Ford's will give the more specialized sensors(such as an SCT X3 tuner for example).

 

Another possibility is the stepper motor in the gauge; they're known to be a problem with the '05s and early 06's.

 

I would also check the upper raidiator hose and make sure it is getting hot. If it is not hot, then your new thermostat is not working right.