Try starting with the A/T in N as well.
But it definately sounds like the safety switch.
As the truck ages, the shifter notch track also wears, and elongates the slot it rides in. This will let the shifter move out of the range of the safety switch, and make the switch look bad. The switch's position is adjustable, so first check if it's simply moved out of alignment...


Don `87 Mitsu 2dr, Rubicon survivor, GModified.