Checked the starter yesterday...no bad bearings letting the armature touch the case, no signs of any arcing or overheating. The old solenoid did seem to engage and kick the drive out fine. The starter is a Powermaster 9400 Ultra Torque:
https://www.jegs.com/i/Powermaster/713/9400/10002/-1
I purchased a new, high quality solenoid from a local alternator and starter rebuilder and everything checked out fine on their test equipment.
I suppose it's possible that the issue originated in the battery. I think I'll put something under it and between the top hold down strap and the battery to give it some cushion. Maybe vibration killed the battery?
I'll have our local rebuilder bench check the alternator, as well.