The suggestions above are good ones. The Solenoid is a great place to start troubleshooting this type of no-start condition, but I would think there would have to be a string of peculiar circumstances for solenoid failure to force an engine to stall all by itself. I think Ignition Switch failure is more plausible given the description of the symptoms. Of particular interest is that you have a no crank condition, though your battery appears healthy by your description of headlamp strength with the key in the START position.
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burned out ignition module?
fried ignition switch? (where exactly is it, BTW?)
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Short of a critical failure that could potentially traumatize the wiring at the bulkhead firewall connector (or somewhere else along Ignition primary), the ignition module can't create a no-crank
condition by itself. I would diagnose that before worrying about spark, which the ignition module can control.
The ignition switch can kill a running engine and keep it from cranking if it has come loose or failed altogether.
Relay? The Starter Solenoid is the best place to start looking, though, like I said, it's a stretch to believe that it forced a stall by itself, though it can sponsor a no-crank condition. I'm unsure of any other relays the YJs may have come equipped with, you may want to think about a wiring diagram. DArksiderz have some for your year here:
Looking at a snippet of the diagram,
It looks like you may want to check and see if the starter solenoid is getting signal on the GREEN wire when the key is in the START position, if not look at the Starter motor relay (perhaps replace), and then work your way back to the ignition switch from there if there's no signal. Either way, you can't worry about idle if you can't even get it to crank, so I would start there if I was you.
Of course it might help if I owned a YJ, but it's not that far removed from a late CJ.