Starter solenoid is not disingaging how do I fix?

So, I bought an F150 1993 300 L6 had no starter.  I purchased starter from auto parts store and installed.  Truck would start but you could hear the starter missing the fly wheel a couple times before it would grab.  After it grabbed the flywheel and started you could hear a loud gravel grinding type noise.  I was told that was the starter still engaged with the flywheel and not retracting.  I shimmed it and eliminated the starter missing the flywheel but gravel grinding noise still there.  Took starter back for a new one (guy told me starter could be bad) auto store seemed surprised they sold me that starter for the F150.  Got a different one, mounted it.  Missing flywheel no longer an issue.  Grinding noise still there.  Tonight, I am going to try and shim again and see if I can get somewhere but don't have hopes up.  Any thoughts for me?