Use paper towels to dry the pork shoulder. If it comes tied in butcher twine, carefully remove it.
In a small bowl, mix together the kosher salt, black pepper, garlic powder, onion powder, oregano, cumin, cinnamon, and cayenne. Rub the mixture all over the pork.
Place the pork in your slow cooker pan with the fat cap on top. Squeeze the fresh orange juice on top. Cover and cook on LOW for 8 hours, until the meat is fall-apart tender.
Use two forks to shred the meat, right in the slow cooker pan. Be careful - the pan and cooking liquids will be very hot. Mix the shredded meat well with the cooking liquids.
Preheat your oven to broil. Line a rimmed broiler-safe baking sheet with foil and spray it with oil. Spread the carnitas on the baking sheet. Some of the cooking juices will remain in the slow cooker - keep it set to WARM. Spray the carnitas liberally with oil and broil them until crispy, 3-4 minutes.
Drizzle the carnitas with the remaining cooking juices. Serve immediately.
Video
Notes
There's no need to let the meat rest before shredding it. You actually want to shred it right away. It's true that normally it's important to allow a roast to rest before slicing it. But here, you want to shred it immediately while it's hot and very soft and mix it with the pan juices. But be careful - the pan and cooking liquids will be very hot.