Combine lime juice, salt, cumin, chili powder, garlic powder, oregano, onion powder, and black pepper to make a rub.
Cut pork shoulder into 3 pieces, removing any excess fat and generously cover with rub.
Place pork shoulder in slow cooker (along with any leftover rub) and top with the orange slices.
Cover and cook on low, 6 - 8 hours or high, 4 hours until very tender.
Discard orange slices; remove meat and shred.
Before serving, in a large skillet, pan fry desired amount of meat over medium heat until it gets a good brown crust over some pieces. (Don't skip this step as it really enhances the flavor.)
Serve pork on warm corn tortillas with desired toppings.