In medium pan, combine chicken, taco sauce, chicken stock, salt, and pepper, and cook on low, covered, approximately 2 hours, or until internal temperature reaches 165 degrees F. Remove chicken from pan and shred using forks. 

Turn heat to medium-high and reduce cooking liquid into thick sauce, cooking 3-5 minutes and stirring occasionally. Remove from heat and combine sauce with shredded chicken. Serve in taco shells with desired toppings.