Drain pineapple, reserving juice; set pineapple aside. In a large resealable plastic bag, combine seasoning mix and reserved juice; add the pork. Seal bag and turn to coat; refrigerate for 15 minutes.
Meanwhile, place the red pepper, onion and pineapple on a double thickness of heavy-duty foil (about 12 in. square). Fold foil around mixture and seal tightly.
Prepare grill for indirect heat. Drain and discard marinade from pork. Grill pork and foil packet, covered, over indirect medium heat for 20-35 minutes or until a thermometer inserted in the pork reads 145° and vegetables are tender. Remove from the grill. Cover pork and let stand for 5 minutes.
Cut tenderloin into strips; place on tortillas. Top with vegetable mixture, cheese and avocado; fold in sides.