Mix the juice of two limes, orange juice, minced garlic, salt, black pepper, ground cumin.
Place juice mixture and pork butt in a large plastic freezer bag.
Marinade for about two hours in the refrigerator.
Place pork shoulder in a large slow cooker with the marinade.Barely cover the pork with water and let cook on medium until meat will shred easy with a fork about 4 to 6 hours.
Remove meat; drain. Shred meat.
Place in warmed tortillas, top with chopped onion and cilantro. Squeeze sliced lime on each taco.