Process lime juice, oil, garlic, salt and red pepper flakes in blender until smooth; pour into shallow bowl. Coat steak with marinade in bowl. Refrigerate covered, turning occasionally, at least 2 hours. Heat broiler or prepare grill. Grill steaks 3 inches from heat, turning once, about 8 minutes for medium-rare. Let stand loosely covered 3 minutes. Warm tortillas wrapped in aluminum foil in hot oven or on grill 3-4 minutes. Thinly slice steak on a diagonal and place on tortillas. Top with guacamole, salsa and sour cream. Roll up and serve hot.