I was at Lowe's today and it left me stranded. Someone stopped and jumped started the truck. He said the truck was vapor locking. I got home and it would not crank. I went to Advance Auto to check battery - battery was good but would not crank. One employee advised the ingition module was bad, but another disagreed and advised this year truck had problems vapor locking. Two people today talked about vapor locking, what do you think.
Vapor locking won't cause it to not crank only to not get fuel (at least thats my understanding of it), however if you are having vapor locking issues you should consider switching to a colder thermostat, the 195 that ford puts in there is excessively hot, they use that mainly to keep emissions down. I use a 170 thermostat in the summer and a 180 in the winter and I use a coolant additive that makes it exchange heat more efficiently.
This is really sounding more like a problem delivering the electric power to the starter, you might want to try removing the wire to the starter and cleaning it with a wire brush on a dremil or electric drill and reconnecting the wire, also it might not hurt to take the starter off and clean where it contacts the bell housing. Either way it sounds like the starter is getting a fraction of the power that it should when things are hot, Also it's possible that the wires appear to be fine but are corroded inside the casing, saltwater from the winter roads can get inside a small nick in the rubber casing and really destroy the wires due to the electric current. If the wires are badly corroded inside the casing they could work fine when they are cold but once they heat up things expand and power doesn't flow as well, unfortunately the only way to check something like that is to replace the wire. Still, the wire doesn't cost much.
Also it could still be an overheating starter, with another tuck hooked up to your electrical system the starter can draw more amperage giving it more torque. You are using the correct battery for your truck right?