Preheat the oven to 325 degrees F. Season the beef with salt and pepper. Heat the oil in an ovenproof pot set over medium-high heat. Sear the beef on all sides. Remove from the heat and drain excess fat.
Combine the barbecue sauce, stock, tomatoes, brown sugar, vinegar, chipotle pepper, cumin and oregano in a bowl. Pour over the beef. Cover and cook in the oven for 3 hours, or until the beef is very tender. Remove the beef from the pot and set in large bowl. Cool a few minutes, and then use two forks to pull the meat into shreds. Skim any fat from the sauce in the pot; stir in the shredded meat. Set the pot over medium-heat and cook until the meat is hot.
Divide and place an equal amount of meat in a row in the centre of each tortilla. Top the meat with the cheese and lettuce. Fold the sides of each tortilla partially over the filling, roll into a cylinder, and serve.