1. Trim any skin or dark or bloody spots off the tuna. Rinse the tuna under cold running water and blot dry with paper towels. Arrange the steaks in a nonreactive baking dish.
2. Combine the basil, garlic, lemon zest, lemon juice vinegar, oil, salt, and pepper in a food processor or blender and puree until smooth. Pour this mixture over tuna and let marinate in the refrigerator, covered, for 30 minutes to 2 hours.
3. Set up the grill for direct grilling an preheat to high. If using a gas grill, place the wood chips, if desired, in the smoker box or in a smoker pouch and preheat until you see smoke. When ready to cook, brush and oil the grill grate. If using a charcoal grill, toss the wood chips, if desired, on the coals. Drain the tuna steaks and arrange on the grill. Grill until cooked to taste, 2 to 3 minutes per side for rare, 4 to 6 minutes per side for medium, rotating the steaks 45 degrees after 2 minutes to create an attractive crosshatch of grill marks. The steaks should be nicely browned on the outside. Test for doneness using the poke method: A rare steak will be quite soft, with just a little resistance at the surface; a medium-rare steak will be gently yielding; and a medium steak will be quite firm. Transfer the steaks to a plate or a platter and let rest for 3 minutes, then server with a salad.