Trim fat from meat. Cut meat to fit into a 3-5 qt crock pot. Sprinkle meat with salt and pepper, rub it in. Sear meat in a skillet on all sides in hot oil. Place meat in cooker, add onion, 1 cup of root beer and the garlic.
Cover and cook on low for 8-10 hours or if needed 4-5 hours on high.
While meat is cooking, whisk all the sauce ingredients together in a saucepan. Bring to a boil, reduce heat. Simmer uncovered, stirring occasionally, about 30 minutes or until mixture is reduced to 2 cups.
Transfer meat to a cutting board or platter. Pull meat apart,(I use 2 forks)into shreds. Transfer the onion to a platter, discard the juices.
Place the meat onto your buns and drizzle the root beer sauce over the meat.
* Do NOT use diet root beer - it really does not work well at all.