Pat the pork shoulder chunks completely dry with paper towels. In a small bowl, combine salt, pepper, oregano, and cumin. Rub the spice mixture evenly over all sides of the pork.

Place the seasoned pork in the slow cooker. Add orange juice, lime juice, smashed garlic, quartered onion, and bay leaves. The liquid should come about one-third of the way up the pork; do not submerge it completely.

Cover and cook on LOW for 8 hours or HIGH for 4-5 hours, until the pork is fork-tender and easily shreds. The meat should practically fall apart when prodded with a fork.

Transfer the cooked pork to a large cutting board with a slotted spoon, leaving the cooking liquid behind. Discard the bay leaves. Use two forks to shred the pork into bite-sized pieces, discarding any large pieces of fat.

Position a rack 6 inches below the broiler and heat to HIGH. Spread the shredded pork in an even layer on a large rimmed baking sheet. Drizzle with 1 tablespoon of the reserved cooking liquid and the vegetable oil. Broil for 4-6 minutes, until the edges are deeply browned and crispy in spots. Watch closely to prevent burning.

Transfer the crispy pork to a serving bowl and toss with a few more spoonfuls of the cooking liquid to keep it juicy. Serve immediately with warm tortillas, diced onion, fresh cilantro, and lime wedges.