Welcome Rick M!

Sounds like Bmorgil has you on the right track. One thing I would add is to look at the starter itself. With 12 volts being applied, the starter Bendix may need to be looked at wile you have it off. The reasoning is that with the boost in volts, the Bendix slams engaged more so then when there is 6 volts. If the Bendix doesn't receive a little lube from time to time, it can hang up against the ring gear and could potentially cause the interface issue. With that being said, I would still follow the advice posted previously because for a ring gear to be that chewed up I would be looking at the tooth count as a starting point.