Trim fat from meat. If necessary, cut roast to fit into crockery cooker. Sprinkle meat with the salt and pepper.
In a large skillet brown roast on all sides in hot oil. Drain.
Transfer meat to a large crockery cooker.
Add onions, the 1 cup root beer and garlic. Cover; cook on LOW for 8 to 10 hours or on HIGH for 4 to 5 hours.
Meanwhile, for sauce, in a medium saucepan combine the 2 cans or bottles of root beer and bottled chili sauce. Bring to boiling; reduce heat. Boil gently, uncovered, stirring occasionally, about 30 minutes or until mixture is reduced to 2 cups.
Add root beer concentrate and bottled hot pepper sauce if desired.
Transfer roast to a cutting board or serving platter. Discard juices.
Using two forks, pull meat apart into shreds.
To serve, line buns, either plain or toasted, with lettuce leaves and tomato slices, if desired. Add meat and onions, then spoon on sauce.