1. Place carnitas and water in a 2- to 4-quart slow cooker. Cover and cook on High for 3 to 4 hours (Low: 6 to 8 hours).
2. Meanwhile, combine jalapeños, onion, radishes, cilantro, sugar and lime juice in a bowl; cover and refrigerate until needed.
3. Remove meat from slow cooker; discard juices. Use tongs or 2 forks to pull pork; spoon pork onto warm tortillas.
4. Strain onion mixture if needed. Top tortillas with onion relish and avocado. Fold top and bottom of tortillas toward center and roll like burrito to serve.