Trim fat from meat. If necessary, cut meat to fit into a 3 1/2- to 5-quart slow cooker. Sprinkle meat with the salt and pepper. In a large skillet brown meat on all sides in hot oil. Drain off fat. Transfer meat to slow cooker. Add onion, the 1 cup root beer, and the garlic.
Cover and cook on low-heat setting for 8 to 10 hours or on high-heat setting for 4 to 5 hours.
Meanwhile, for sauce, in a medium saucepan stir together the 3 cups root beer and the chili sauce. Bring to boiling; reduce heat. Simmer, uncovered, stirring occasionally, about 30 minutes or until mixture is reduced to 2 cups. If desired, add root beer concentrate and hot pepper sauce.
Transfer meat to a cutting board or serving platter. Using a slotted spoon, transfer onion to serving platter; discard cooking juices. Using 2 forks, pull meat apart into shreds.If desired, line buns with lettuce leaves and tomato slices. Place meat and onion on rolls. Drizzle meat with some of the root beer sauce. Makes 8 to 10 servings.