1989 Ford Ranger Starter keeps running (engaged)

Did you check for voltage at the starter. You should be able to test at the starter relay on fender apron. Red wire is hot all the time. Red wire with lite blue tracer should only be hot with key in the crank position. The black cable carries voltage to starter only when relay is energized, with key in the crank position.

Let us know about the voltage checks. Make sure tranny is out of gear and functional parking brake is set.