Prepare the grill for direct cooking over medium heat (350° to 450°F).
In a medium bowl mix the glaze ingredients. Transfer ½ cup to a small bowl. Remove the membrane from the back of each rack of ribs and cut each rack crosswise in the middle to create two smaller racks.
Season the half racks all over with the remaining glaze in the medium bowl, about 2 tablespoons per half rack, putting more of it on the meaty sides. Using eight 18-by-24-inch sheets of heavy-duty aluminum foil, double wrap each half rack in its own packet and seal tightly. Allow the racks to stand at room temperature for 30 minutes before grilling.
Place the foil-wrapped racks over direct medium heat, close the lid, and cook for 50 minutes to 1 hour (the smaller half racks will be done first), turning the packets over once or twice, making sure not to pierce the foil.
Remove the packets from the grill and let rest for about 10 minutes. Carefully open the foil packets, remove the racks, and discard the rendered fat and foil. Grill the racks over direct medium heat, with the lid closed, until sizzling and lightly charred, 6 to 8 minutes, turning once or twice and basting with the reserved ½ cup glaze. Remove from the grill and let rest for about 5 minutes.
Cut the racks between the bones into individual ribs and serve warm.