Place London Broil in a large zipper-topped plastic bag.
In a small bowl, whisk together next 6 ingredients (garlic through broth) until well blended; pour mixture over meat.
Seal bag, squeezing out excess air. Turn bag several times to coat meat evenly then refrigerate for 2 hours.
Empty the contents of the bag into a large crock cooker.
Cover and cook on LOW for 6 hours or on HIGH for 3 hours or until beef is fork-tender; remove from crock cooker and slice.