Replaced washer drive block (or tub drive block). This requires removing the cabinet, the agitator, and the tub to get down to the drive block. It didn't seem that worn, but since it's not an expensive part and I had the washer apart already, I ordered it. Even once it came, I wasn't convinced it was that different from the part I'd removed. But since I had it, I put it on and reassembled the washer. Ran a loss and the tub remained upright.