Let me say at the outset, I've never rehabilitated an orphaned woodchuck or any other marmot, nor have I kept any as pets. However, I do know a bit about marmots and people have asked for suggestions on how to care for orphans. I'm reproducing advice from those that have kept them as pets.
If you find an orphan and the marmot has fur and its eyes are fully open, it probably has a good chance of surviving--they're pretty hardy. People have told me that they've fed orphans with goat's milk and bread soaked in cow's milk. I'm not an animal nutritionist, but they both seem like a good start. Keep the pup(s) warm and dry, give them something to nest with (like old socks and paper towels), and make sure they get liquids and nutrients--especially protein.
Marmots are generalist herbivores. Start them on fresh fruits and vegetables as soon as possible. Apples, lettuce, carrots, celery...you're going to make a monster that will search out and destroy gardens so don't worry about doing it in style. Marmots really like legumes; alfalfa might be a good thing to feed them if you've got access to fresh or dried alfalfa. Marmots love dandelions and cow parsnip. If they're getting fresh vegetables, they'll probably not need water. If you feed them rodent chow (they get obese on rodent chow), you'll need to give them water. Some of the marmots we've hibernated really liked water (we called them "drinkers") while others didn't. Marmots are like us--variable in their behavior and tastes.
Marmots are often infested with mites and fleas. Bathing a marmot is OK--as is dusting it with flea powder. Be careful not to poison them though.
If you're aiming to rehabilitate it, I suggest moving it outside into a semi-natural enclosure. A wood crate with a "door" cut into it, filled with old socks, and paper towels (for bedding) might be a good home. The idea is to make something that will protect it against predators (cats, dogs, raptors) until it's a bit stronger and more sure of itself. Keep feeding it at the box, but let it pretty much explore on its own. A female may ultimately settle in the area--males may trundle off and settle farther away. Woodchucks typically disperse near the end of their first summer and yellow-bellied marmots typically disperse during their second summer.
If you want a pet, get them young--within a few days of emergence (and therefore just around weaning). Females probably make better pets than males, but I've heard some good stories of males kept as pets. If you're going to keep it as a pet, remember: marmots hibernate! So, when you wake up one winter morning to find a cold and barely breathing marmot--it's probably fine. A family in Wyoming that has a pet yellow-bellied marmot finds that it goes into torpor when they go off on vacation and don't bring it along! And, it can go into torpor any month. Great pet! Marmots are rodents so make sure they've got something to gnaw on to keep their teeth worn down. Marmots also love to climb into dark corners, desks, and drawers filled with underwear--so be prepared. I don't know how to toilet train them, but having a "home nest box" might be a good thing. Some people have written in to say that their pet marmots have toilet trained themselves. In nature, some species often defecate just outside their burrow and then use the feces to make a plug when they hibernate.
Write me with your successes
and failures and I'll periodically update this brief guide.