Place roast into crock cooker. Season with sea salt and freshly ground black pepper to taste. Add garlic, rosemary and onion. Pour on barbecue sauce and vinegar. Cover with the lid.
Cook on low for 6-8 hours, or until the beef is cooked through and shreds easily.
Shred beef with a fork in the crock cooker. Cook for 1 additional hour on low. Serve hot.