Had this on my '96 a few years ago. In my case, I couldn't turn off the engine!
The ignition lock pushrod that activates the ignition switch (near the lower base of the
top of the steering column) was the culprit.
Remove the steering column cowl, and (maybe) the instrument gauge pack. You'll see
part of the rod on top of the steering column, disappearing deep into the dash.
Be sure that nothing is interfering or fouling the rod. Then spray it liberally with silicone
lube (let the lube dribble down the rod, onto the ignition switch) while jiggling it with a
pair of needle nosed pliers, and also try working the ignition key On & Off a few times
until everything works its way loose.