In a small bowl, mix chili powder, cumin, smoked paprika, salt, pepper, and red pepper flakes.
Rub the spice blend all over the chuck roast.
Place the seasoned roast into the slow cooker.
Top it with garlic, onion, diced tomatoes, and beef broth.
Cover and cook on low for 8 hours, or until the beef is tender and falls apart easily.
Remove the beef and shred it using two forks.
Return the shredded beef to the slow cooker, add lime juice, stir, and let it soak in the juices for a few minutes before serving.