Soak 3 cups wood chips in water according to the package directions or for 1 hour. Meanwhile, combine the 2-1/2 tablespoons brown sugar, the paprika, onion powder, celery salt and 4 teaspoons each salt and pepper. Run a paring knife down the length of the bone side of each rack to split the skin, then season all over with the rub.
Remove the grill racks. Arrange a drip pan or disposable baking sheet on one side of the grill to catch drippings. Drain the wood chips and transfer to a 2-foot-long sheet of heavy-duty foil. Wrap tightly, creating a secure pouch, then poke 10 holes at the top to create a smoker box. Place the pouch on the other side of the grill, opposite the drippings pan. Set the grill racks into place, close the grill and preheat to 325 degrees .
Working quickly, arrange the ribs, meaty side up, over the drip pan. Grill, covered, turning occasionally, until the meat shrinks away from the bone and is fork-tender, about 3-1/4 hours.
Meanwhile, in a small saucepan, combine the remaining 1/2 cup brown sugar, the ketchup, honey and vinegar over medium-high heat and season with salt and pepper. Bring just to a boil, then lower the heat to medium-low and simmer, stirring occasionally, until smooth, about 5 minutes.
Brush the ribs all over with the sauce, arrange meaty side up and grill, turning once, at 200 degrees for 20 minutes. Cut the racks into individual ribs and serve with the sauce on the side.