1993 Other Jeep Models Grinding starter noise, but only when

I have a 1993 Jeep Grand Cherokee, 4.0L 6cyl. Automatic. A couple days ago my Jeep wouldn't start, no click, no clack, no whuur, nothing. I figured, dead battery. Well when the jumper cables were applied, same thing. Nothing. A light tap on the starter did the trick. So, I replaced the starter solenoid. The starter itself seemed ok. With the solenoid off, the starter gear slid back and forth effortlessly, the connector arm seemed intact and working, and the teeth looked great, so I left the motor alone. However. When I got everything back together, the car starter wonderfully, until. I put it in gear, I then heard the terrible grinding gears sound, as if the starter gear was trying to engage. When I shift back into Park the grinding goes away. In neutral it's really bad.
When I pulled the starter back out, I saw evidence of metal shavings to lead me to believe this was indeed the noise I was hearing.
So, why does it start wonderfully and make no noise in park, until I shift into gear? It really gets bad when I shift into neutral. It will go away when I shift back into park.
One thought is that maybe the neutral safety switch has broken or gone bad?
Another thought is that, even though I thought the starter motor looked fine to me (it starts the car) maybe it's not fine and needs to be replaced?
I'm looking to avoid a new starter motor if there is something else that you can think of that would cause this to happen. Why would it only grind when it's in gear or neutral? And not in park?

Thanks