Working with one rack of ribs at a time, slip a knife between the bones and thin membrane to loosen, then grab the membrane with a paper towel and pull it off. Put the ribs on a baking sheet and rub the spice rub on both sides. Cover with plastic wrap and refrigerate 6 hours or overnight.
Prepare a grill for indirect grilling: For a gas grill, preheat to medium high; after about 10 minutes, turn off one or two of the middle burners and turn the remaining burners down to medium low. For a charcoal grill, light the charcoal; once the coals have just ashed over, carefully push them to opposite sides of the grill and set a drip pan in the middle to avoid flare-ups.
Place the ribs bone-side down on the cooler part of the grill (overlap the rib racks slightly to fit, if necessary). Cover and cook, rotating the position of the rib racks once halfway through, until the meat is tender and pulls back from the ends of the bones, 2 hours to 2 hours 15 minutes. Flip and move to direct heat and cook, uncovered, until charred in spots, about 5 minutes. Transfer to a cutting board and cut into pieces.