It seems this vehicle has had this problem for a long time. I changed the plugs at 50,000 I think. Now has 90,000. It runs rough at idle when in drive and stalls periodically. V6 Any suggestions as to troubleshooting would be helpfull.
Do you happen to have access to a scan tool with a datastream mode so you can monitor things like O2 sensor voltages and fuel trim values real time? If so, monitoring the Bank 1 and 2 Sensor 1 voltages as well as the short term fuel trims when the truck is idling roughly would help you understand if this is likely a vacuum leak issue or something else. My bet would be on a vacuum leak, either intake manifold, PCV elbow, or and EGR valve that is not closing fully at idle.
You could check for relay coil trigger to the starter relay, and if is present, then reinstall the relay and check for trigger to the starter solenoid. This will probably require someone skinny to slide under the Explorer and snake their arm around to the starter motor assembly while another person turns the key to start. Of course the person under the truck will want to make sure as the engine turns over and possibly starts nothing will be near the belts.
You could also try shifting to neutral and starting it in the event the problem is with the transmission range sensor, but I don't think you'd even get the single click if that were the case.
A series of clicks would indicate low voltage as the starter attempts to engage but drops out due to low voltage, the open load lets a surface charge build back up on the battery, the relay/solenoid engage again, and the cycle repeats. A single click is indicative of other issues such as worn brushes in the starter.