Grate the zest of the orange into a 5 to 6-quart slow cooker, then squeeze in the juice (you should get at least 1/2 cup). Stir in the garlic and vinegar.

Season the pork with the chili powder, oregano, cumin, and 1/2 teaspoon salt. Add to the slow cooker and toss to coat. Cover and cook until the meat is tender and shreds easily, 7 to 8 hours on low or 4 to 5 hours on high.

Twenty minutes before serving, make the pickled onions. Thinly slice the onion and toss with the lime juice and 1/4 tsp each salt and pepper. Let sit, tossing occasionally, until ready to serve.

Using two forks, shred the meat and toss with the cooking juices. Pile onto tortillas, top with the onions, cilantro, and avocado, if using.