1. On bony side of ribs, loosen edge of membrane with a table knife. Pull off membrane, using a towel. Cut racks in half. Smear all over with rub. Chill airtight at least 4 hours or overnight; let stand at room temperature for last hour.
2. Meanwhile, soak 2 cups applewood chips in water 30 minutes. Scrunch 6 sheets (1 1/2 ft. long) of foil into logs 9 in. long. For gas, set a drip pan on center burner (the indirect heat area), fill it halfway with very hot water, then heat the other burners to low (275° to 300°). For charcoal, ignite 50 briquets in a chimney. Bank evenly on opposite sides of firegrate, leaving a cleared area in center for drip pan; fill pan halfway with very hot water. Let coals burn to low. Add 5 briquets to each mound of coals every 30 minutes, starting when ribs go on grill; if fire gets too hot, partially close vents on lid.
3. Set racks upright over drip pan facing one another and wedge foil logs into tops of racks (in between bones) to bridge them and hold them in place. For gas, drain half the wood chips and put in a smoker box, if grill has one; or seal chips in foil, poke quarter-size holes all over it, and set over direct heat (in foil, chips may take awhile to start smoking). For charcoal, drain half the wood chips and sprinkle evenly over coals.
4. Grill ribs, covered, 1 hour. Drain remaining wood chips and add as before. Cook until meat is tender when pierced and shrinks 1/2 in. from bone tips, 2 to 3 hours.
5. Pour 1 cup sauce into a bowl. Remove foil from ribs and set ribs flat, bony side up, over indirect heat. Brush with 1/2 cup sauce. Cook until sauce sets, 5 minutes. Turn, brush again, and cook 3 minutes more.
6. Let ribs sit on a board, tented with foil, 10 minutes. Serve with remaining sauce. Chill extra sauce 1 week, or freeze.
*Ask a butcher to trim ribs, or do it yourself:
On bony side, trim flap of meat from center, flush with bones. Cut rack lengthwise between wide rib section and chewy, narrow rib tips. Save trimmings for soup.
Note: Nutritional analysis is per serving with half the sauce.