Soak eight 6- to 8-inch-long wooden skewers in enough water to cover for 30 minutes; drain. Thaw shrimp, if frozen.
Cut pineapple lengthwise into 1/2-inch-thick slices. Chop one of the slices to measure 1/2 cup; set aside for relish. Halve each pineapple slice crosswise (you should have 8 planks of pineapple). Using 1 tablespoon of the melted butter, brush both sides of each pineapple plank; set aside.
Peel fresh shrimp, if using, leaving tail intact, if desired. Devein shrimp. For marinade, in a medium bowl combine the remaining 2 tablespoons melted butter, the barbecue sauce, chipotle peppers, and the garlic. Stir in shrimp. Cover and marinate in the refrigerator for 30 minutes, stirring occasionally. Remove shrimp; discarding marinade. Thread 1 jumbo or 3 large shrimp onto each skewer; set aside.
For relish, in a bowl combine the reserved 1/2 cup chopped pineapple, cucumber, jicama, lime or lemon juice, and salt. Cover and set aside until serving time.
Place shrimp kabobs and pineapple planks on the rack of the grill directly over medium heat. Grill until shrimp are opaque and pineapple is heated through, turning once halfway through grilling. (Allow 10 to 12 minutes for jumbo shrimp, 6 to 10 minutes for large shrimp, and 6 to 8 minutes for pineapple planks.) If desired, brush each shrimp kabob with additional bottled barbecue sauce during the last 1 minute of grilling.
To serve, stir snipped cilantro into the relish. Place pineapple planks on a serving platter. Spoon some of the relish over the pineapple planks; top each with a shrimp skewer. Makes 8 servings.