In a small bowl, combine chili powder, cumin, garlic powder, onion powder, paprika, salt, and black pepper to create a spice rub.
Rub the spice mixture all over the flank steak, coating it evenly on all sides.
Place the seasoned steak in the slow cooker and add beef broth and diced green chiles.
Cover and cook on low for 7-8 hours or on high for 4-5 hours, until the meat is very tender and shreds easily.
Remove the steak from the slow cooker and shred it using two forks, then return the shredded meat to the slow cooker.
Stir in lime juice and let the meat soak up the juices for about 10 minutes.
Warm the tortillas and serve the shredded beef with your favorite taco toppings.