On GM RWD vans we've had 3 come in already with the same issue, starter self-engages on it's own in the run position. It will shut off in drive and reverse, but shift into neutral or park and it grinds away. Turns out that the issue was in the neutral safety switch as it contains many circuits, and it was shorting internally feeding power to the starter. A new one fixed all 3 vans/trucks, but was difficult to replace because the connectors were melted to the switch. I can't say for sure that's what's causing your issue, but something's powering up the starter when it shouldn't be. Does it stop grinding in drive or reverse?
