Trim fat from meat. If necessary, cut meat to fit into a 3-1/2- or 4-quart slow cooker. In a small bowl combine the 1/2 teaspoon lemon-pepper seasoning and the dry mustard. Sprinkle mixture evenly over meat; rub lightly with your fingers. Place meat in slow cooker. In a small bowl combine broth, lemon peel, lemon juice, rosemary, and garlic. Pour over meat in cooker.
Cover and cook on low-heat setting for 8 to 10 hours or on high-heat setting for 4 to 5 hours.
In another small bowl stir together yogurt, cucumber, and the 1/4 teaspoon lemon-pepper seasoning. Set aside.
Transfer meat to a cutting board, reserving cooking juices in cooker; cool meat slightly. Using two forks, shred meat, discarding fat. Place meat in a medium bowl.
To serve, open pita bread halves to form pockets. Place a lettuce leaf in each pita half. Spoon meat into pita halves. Top meat with yogurt mixture and chopped tomato.