1. To brine ribs: Dissolve salt and sugar in 4 quarts cold water in stockpot or large plastic container. Submerge ribs in brine and refrigerate 1 hour until fully seasoned. Remove ribs from brine and thoroughly pat dry with paper towels.
2. While ribs are brining, cover 2 cups wood chips with water in medium bowl; soak wood chips for ½ hour, then drain and set aside. Combine spice rub ingredients in small bowl. When ribs are out of brine and dried, rub each side of racks with 1 tablespoon spice rub; refrigerate racks 30 minutes.
3. To barbecue ribs: Ignite one burner on and the other(s) off mimics the indirect heat method on a charcoal grill.. Place wood chips in a small disposable aluminum pan and place over ignited burner. Turn all burners to high, close lid, and heat grill until chips smoke heavily, about 20 minutes. (If chips ignite, extinguish flames with water from squirt bottle.) Scrape grill grate clean with wire brush; turn off burner(s) without wood chips.
4. Arrange ribs on cool side of grill and cover (grill temperature should register about 275º F on grill thermometer). Cook for 4 hours, until meat easily pulls away from bone, flipping rib racks, switching their position so that rack that was nearest fire is on outside, and turning racks 180º F every 30 minutes. Transfer ribs to cutting board, then cut between bones to separate ribs; serve