Prepare a grill to medium heat (350 degrees).
Starting at the bony underside of the rack, slip a small thin knife under the membrane on the bones. Loosen about 1 inch of the membrane. Grab the loosened membrane with a paper towel and pull along the length of the rack to remove the membrane. (You may have to do this a couple of times until most of the membrane is removed.) If needed for space on the grill, cut each slab in half between two ribs.
In a small bowl, mix chili powder, garlic salt and 1 teaspoon ground chipotle. While grill is heating, rub mixture all over ribs and let stand for 15 to 30 minutes.
Pour 1/4 cup of cola into a bowl. Place ribs bone-side-down on grill over indirect heat. Cover and cook for 1 1/2 to 1 3/4 hours, brushing ribs with cola on both sides about every 20 minutes, until rib bones shrink to expose bones by about a 1/2 inch at ends.
Meanwhile, in a small saucepan, combine barbecue sauce with remaining 1/3 cup cola. On the stove, simmer over medium-low heat, 10 to 15 minutes, stirring occasionally, until reduced to 3/4 cup. Stir in remaining 1/2 teaspoon ground chipotle.