You should read this thread first...
http://www.explorerforum.com/forums/showthread.php?t=287097
If that's not your problem, get under the truck with a voltmeter and test to see if there is any power at the small wire going to the starter. That's the wire that signals the solenoid to connect the battery cable with the starter, providing the power.
Here's the electrical path for the starter relay wire:
There is a fuse (F2.29) in the central junction box that is hot when the ignition switch is in the start position. This power source then goes to the Digital Transmission Range sensor (which acts as a neutral safety switch). If it passes thru this switch, it then goes to the starter relay to power it up. The PCM provides the ground for this starter relay.
Here's the path for the starter signal wire:
The battery junction box has a 50amp fuse (F1.12) that sends a power source to the starter relay. When the relay is activated (via the system mentioned above), that power comes out of the relay and to the starter solenoid. There, it activates the solenoid, which connects the big battery power to the starter.
The big battery power circuit is like this:
The battery has 2 grounds. One goes to the body near the battery. That grounds pretty much everything but the starter. The big negative battery cable goes to the block, and that provides a ground path for the starter. The positive battery cable goes straight to the starter motor.
In the thread mentioned at the top of this page, the 4.6 engines have this + battery cable, an AC wire, and a few other wires (probably including the starter solenoid wire!) going thru a harness that passes over the plastic oil filter drain spill. The plastic drain cuts thru the protective plastic and eats into the wires. Usually, the first problem people have is their AC quits working. But the starter cable wires can get cut thru, as well as the starter solenoid wire if its in there, which I think it is.
Check this first! Under the car, look upward straight in front of the front differential, and look at the wiring harness that's passing right by the plastic oil drain. See if it's damaged. If it is repair it.
If that's not your problem, start testing at the starter relay. Try a known good relay for a quick check of that, and if not, start testing to see if the power is coming into and going out of that relay as it should.
The DTR sensor (park/neutral switch) is not known to be a big issue in these vehicles, as far as I know. So don't throw parts at this truck. Throw diagnostic time at the electrical system. All you need is a voltmeter and a wiring diagram.