You may exit and re-enter the U.S on a TN visa given that the visa has not expired, and you are currently working for the same employer. Sometimes they will check if you are with the same company as the one that issued you the letter.
See here for more information:
http://www.tnvisabulletin.com/nafta-tn-blog/2010/5/3/vacations-while-on-a-tntd-visa.html
