Remove meat from bone; discard bone. Trim fat from meat. Cut meat into 2- to 3-inch pieces; place in a 3 1/2- or 4-qt. slow cooker. Stir in onion, orange juice, cumin, salt, oregano and cayenne.
Cover and cook on low 8 to 10 hours or high 4 to 5 hours. Remove meat from cooker. Shred meat using two forks. Stir in enough cooking liquid to moisten.
For lime crema, remove 1 tsp. zest (set aside) and squeeze 2 Tbsp. juice from lime. In a small bowl combine lime juice, yogurt, and dash salt.
Serve meat in tortillas with cabbage, red onion, salsa (if desired), and lime crema. Sprinkle with lime zest.
1 vegetable, 1 1/2 starch, 3 lean meat, 1/2 fat