V.J.
Two yr olds have great imaginations-- is it possible that something in the room could be creating a "scary" shadow? Maybe a new piece of furniture or something like that??? Maybe he remembers something that was scary for him that happened in his room like tripping and scraping his knee. A few weeks ago my 2 yr old son bumped his head on the tub faucet. He wasn't badly hurt by any means but for days after that he cried whenever I put him in the tub. Could also be that your son had a bad dream. Is he allowed to watch anything on TV that could cause him to have upsetting dreams? Has anything in his bedtime routine changed, has he started daycare, or are you potty training him? He could be stressed out or feel unstable if there have been any changes.
Those are just a few ideas that come to mind. Honestly, I wouldn't let my son cry it out if he was so upset he started shaking. For whatever reason he is getting so upset, he needs you to be there to comfort him and reassure him that he's safe. I understand letting him cry it out if he's just being ornery but it sounds like this is different. If it's feasible, maybe you could sleep next to his crib/bed for a night or two to see things from his perspective and to affirm to him that there is nothing to be afraid of. I'm not sure if this was any help but I hope everything works out. :)