Preheat the oven to 300 degrees F. Place each rack of ribs on a doubled sheet of aluminum foil.
In a bowl, whisk together the salt, paprika, mustard, garlic powder, pepper, onion powder and chipotle chili powder. Rub the mixture all over both sides of the ribs, then wrap the ribs up in the aluminum foil. Place the racks on a baking sheet and put them on the center rack in the oven. Bake for 2 1/2 hours.
After 2 1/2 hours, gently pour the juice in the foil packets into a large bowl or measuring cup. Whisk in 3 to 4 tablespoons of BBQ sauce.
Heat the grill to the highest setting and remove the ribs from the foil. Once the grill is hot, place the ribs on the grill and cook for 10 to 12 minutes, until they are charred and slightly crisp on the outsides, basting and rubbing the juices/BBQ sauce mixture on top. Let cool slightly before serving with lots of BBQ sauce.