Spoke too soon, while warming up the car on lunch it began to change gears again. It lunged forward slightly (while in "P") and then made a rattling sound when I hit the brakes. I put it in "R" and it did nothing.
After two minutes of warming it up, it ran fine and I drove away.
I'm going to take it to the nearby transmission shop tomorrow, but I'd love some advice on what to check first.
1976 230.4 W115