I tend to think you're right about the linkage needing to be replaced. Here (attached) are the components diagrams and the installation procedure (which includes how to adjust the wiper arms). If your wiper arms are correctly adjusted per these specs, it's probably the wiper link mechanism that's over-traveling.
My guess is that the wipers are staying 'in bounds' in dry conditions because of the extra friction of the dry windshield.
My guess is that the wipers are staying 'in bounds' in dry conditions because of the extra friction of the dry windshield.