Need some ideas,
1989 Silverado 4x4 , Auto. 5.7 liter
Truck won't start but starter continues to turn even after key is turned off
and removed from ignition switch.
I'm thinking ignition switch, but would like some input before I take the thing apart

 

Sounds to me like it is ignition switch

 

if you were to shift the trans to drive(keep foot on brake), does it stop cranking. If yes, suspect the ignition switch. to be sure test the switch

locate the wire that feeds the starter ciruit from the ign switch; test for power out of the ignition switch with a voltmeter.

 

Could also be a stuck starter solenoid.