Trim away fat from pork and cut into 2-inch cubes. Stir together dry seasonings in a small bowl; sprinkle over pork, turning to coat all sides. Place tomatoes, lime juice, 1 onion and 1 clove garlic in a large slow cooker; top with pork and raisins.
Cover and cook on high for 5 hours or low for 10 hours. Remove meat from slow cooker with a slotted spoon and place in a large bowl; keep warm. Pour juices from slow cooker into a medium saucepan. Add remaining onion and garlic to saucepan with dried pepper.
Bring to a boil; reduce heat and simmer for 20 minutes, stirring occasionally. Let cool slightly then puree in a blender or food processor.
Shred pork using 2 forks and stir in half the sauce. Spoon into tortillas, if desired. Serve with avocado, cilantro and remaining sauce.