1. Place chicken in a 4- to 5-quart slow cooker. Season with oregano and cumin. In a bowl, stir together soda, tomato sauce and chipotles. Pour into slow cooker, cover and cook on HIGH for 6 hours or LOW for 8 hours.
2. Heat oven to 350 degrees . Remove chicken to a bowl and pour liquid from slow cooker into a saucepan. In a small bowl, blend sugar, cornstarch and salt. Stir into a saucepan along with vinegar. Place over medium-high heat and bring to a boil.
3. Reduce heat to medium and simmer 3 minutes, until thickened and clear. Shred chicken with 2 forks. Pour sauce over chicken and stir to combine. Let stand, covered, for 10 minutes.
4. Make sandwiches: Place a slice of cheese on each roll. Bake at 350 degrees for 2 to 3 minutes or until cheese melts. Divide chicken among rolls. Add pickle slices and, if desired, red onion.