I have a 1991 6 cyl automatic 4.9L. Have replaced solenoid, had starter checked, and is ok. Replaced ignition switch on top of steering column. Starter continues to spin while engine is running. Have to disconnect negative battery cable from post to stop the starter from spinning. With ignition switch disconnected, if I try to put ground cable back onto negative battery terminal, the starter will still spin. What can this be?

 

sounds like a short to power somewhere between the ignition and the starter.
~Josh

 

You have a bad solenoid. The parts store solenoids are junk. I went through ten of them before I finally went to the junk yard to get a Motorcraft part. No problems since. If you whack the solenoid when it sticks it will stop cranking, but use something other than your hand like a wrench or something because it hurts.