Cut the steak into 3-inch strips. Combine the rub (brown sugar and pepper), and then rub the steaks well with the mixture.
Add all the sauce ingredients to your slow cooker and stir to mix well. Add the steaks and turn to coat with the sauce.
Cover the slow cooker, and cook on high for 1 hour. Then reduce to low and cook 7 or 8 additional hours, until the beef is tender. Remove the steak from the slow cooker (leave the sauce in there), and place it on a cutting board. Shred the steak with two forks, and then return the shredded beef to the slow cooker. Stir to coat with the sauce.
Spoon 1/2 cup of the shredded beef onto the bottom of each half of the buns. Top with onion rings and pickles, if desired. Place tops of the buns on top of the beef, and serve.