In a small bowl, combine the first five ingredients. Cut roast in half; rub spice mixture over meat. Transfer to a 3 quart slow cooker. Top with chilies, onion and garlic. Pour broth over meat. Cover and cook on low for 8-10 hours or until meat is tender.
Remove meat from slow cooker; shred with two forks. Skim fat from cooking juices. Return meat to slow cooker; heat through. Using a slotted spoon, place 1/2 cup meat mixture on each taco shell. Top with tomatoes, lettuce and cheese or toppings of your choice.