Jump power on the solenoid on the passenger inner fender. This will prove that the batteries and starter are good, and that the 'start' circuit from the ignition switch to the solenoid is bad.
You could have an issue with the ignition tumbler itself or the wiring to the solenoid.
I recall there being a passive theft device somewhere under the dash that interrupts the start circuit. It happened on a '95 psd we had. It failed, don't know how it worked. We would place the key in the 'on' position then jump the solenoid under the hood to get home. I remember it being a black box a little larger than a billfold. It was located near the steering column.