In a small bowl, mix together the chili powder, ground cumin, oregano, kosher salt, black pepper, and chipotle powder.
Add the pork to a 6 quart slow cooker, and sprinkle the spice mixture over top. Stir to evenly coat the pork in the spices.
Sprinkle the white onion and garlic over the pork.
Pour the orange juice and lime juice over the pork.
Cover and cook on LOW for 8-10 hours, or on HIGH for 5-6 hours (or until the pork is tender and easily shreds with a fork). Stir once halfway through cooking
Shred the carnitas in the slow cooker using two forks.
Using a slotted spoon, transfer the carnitas to a baking sheet and spread it out into a single layer.
Broil the meat in the oven for about 5 minutes (or until the tops are golden brown and crispy).