Pierce chicken on both sides with fork; brush both sides with salad dressing. Spray grill rack with no stick cooking spray. Using direct heat cooking method, arrange chicken breasts 4-5 inches over medium coals. Grill, covered, 5 minutes. Turn chicken; brush with salad dressing. Continue grilling until juices run clear (3-5 minutes). Cool; cut into thin strips (1x1/2x1/4-inch). Cook pasta in boiling salted water (4 quarts water, 2 teaspoons salt) until just tender (12-14 minutes); drain. Rinse with cold water, drain. In large bowl, combine chicken, pasta, celery, green onion and grapes. In small bowl, combine oil and 1 1/4 teaspoon thyme; toss into chicken mixture. In small bowl, combine mayonnaise, milk, honey, 1 tablespoon thyme, mustard and salt. Refrigerate salad and dressing separately, covered, overnight. Several hours before serving, fold dressing into salad. Refrigerate, covered.