Dear Liz,
The woodchuck, or groundhog, like most mammals, (other than bats) is repelled by the smell of ammonia. A strong ammonia odor will make most animals flee even the most comfortable of lodgings.
Try sliding a bowl of ammonia, or several ammonia-soaked sponges under the garage. But do not work too near the woodchuck’s entryway; though they are adorable and can seem quite tame, woodchucks have impressive teeth and jaws. After the woodchuck is gone, you might want to install hardware cloth across his entryway.
Woodchucks are inordinately fond of clover, and seem to prefer it to vegetables. A patch of clover planted away from your garden may help distract future woodchucks and keep them away from your vegetables.
Good luck!
Environmentally yours,
Ellen