BLEND first 4 ingredients in blender until smooth. Pound chicken to flatten to 1/4-inch thickness; place in large bowl. Add guajillo pesto; toss to coat. Refrigerate 30 min. to marinate.
HEAT large skillet on medium heat. Add chicken in batches; cook 8 min. or until done, turning occasionally. Remove from skillet; cut chicken into strips.
TOP tortillas with 2% Milk Singles and chicken; fold in half. Cook, 3 at a time, in large skillet on medium-low heat 2 min. or until Singles are melted, turning once. Transfer to plate; top with lettuce and tomatoes. Serve with sour cream.