Prepare the rub by mixing all the ingredients until well combined.
Place the pork in a large shallow dish. Add the rub and cover all the meat with it. Cover the pork with plastic wrap and place in the refrigerator for 8 hours or overnight.
When ready to cook, preheat oven to 300°F. Remove pork from the refrigerator and discard juices. Remove excess rub. Place the pork in a roasting pan and cover with aluminum foil. Cook in the oven for four hours. Remove the foil and cook for two more hours or until the meat is tender. While the pork is cooking, baste with juices every hour. Shred the pork into smaller pieces.
Prepare the pickled onions. Combine all the ingredients except the onion in a bowl. Mix until the sugar and salt dissolve. Add the onion. Let sit at room temperature for 1 hour.
Prepare the tacos. Warm the tortillas and place a piece of carnitas on top. Top with pickled onions, cilantro, jalapeños, and cheese. Serve immediately.