1Mix lime juice, oil and chili powder in shallow glass or plastic dish or heavy-duty resealable plastic food-storage bag. Add chicken and onion; turn to coat with marinade. Cover dish or seal bag and refrigerate at least 1 hour but no longer than 24 hours.
2Brush grill rack with vegetable oil. Heat coals or gas grill for direct heat.
3Remove chicken and onion from marinade; reserve marinade. Cover and grill chicken and onion 4 to 6 inches from medium heat 8 to 10 minutes, brushing frequently with marinade and turning once, until chicken is no longer pink in center. Discard any remaining marinade.
4Divide chicken and onion among tortillas, placing in center. Fold one end of tortilla up about 1 inch over chicken mixture; fold right and left sides over folded end, overlapping. Fold remaining end down. Top with salsa and guacamole.