Cut pork into cubes; place in crockpot. Chop onion, put on top of pork, pour in broth and juice of one orange. 

When pork is cooked, remove from slow cooker; shred pork with two forks, and return to slow cooker. Add tomatoes (with the juice), chipotle peppers, coriander, cumin, salt, pepper, and the juice from half of one lime. Give it a stir, and cook for 1 more hour.  

Serve with corn tortillas, lettuce, cheese, fresh watermelon and remaining lime wedges.