I highly doubt it is caster and here are two reasons why: 1) Alignment problems don't usually cause shaking issues, usually they cause handling issues and abnormal tire wear. 2) There is no reason for the caster to suddenly change and cause a shaking problem, unless you hit something really hard.
Go to a tire shop and have them spin all four wheels/tires on a wheel balancer. This way you will be able to tell if the tires are in need of balancing of if you have a bent rim. You don't need to go to a dealer for this...... any reputable tire shop can do it for you.
One other possibility is: If you guys got snow recently then a build-up of ice and snow on your wheels can cause this problem also.
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