Prepare the grill for direct cooking over high heat (450° to 550°F).
To kill each crab, place it on its back and hold it down with a large cleaver. Use a hammer to tap the top edge of the cleaver and cut the crab in half lengthwise all the way through. Remove and discard the dangling tail flaps and mouth parts. Turn the crab over. Pull off and discard the top shell. Remove and discard the gills. Rinse each half of the crab under cold running water and use your finger to scoop out and discard the dark viscera. Slice each half into five pieces by cutting between each leg and through the body. Using the hammer again (or a nutcracker or crab cracker), crack each section of each leg to make eating easier after the crabs are cooked.
In a 12-inch cast-iron skillet combine the butter, garlic, lemon zest, lemon juice, red pepper flakes, salt, and pepper. Mix well.
Place the skillet over direct high heat and cook, with the lid open, until the butter melts and the garlic turns golden, 2 to 3 minutes. Add the wine and cook until it comes to a boil. Remove the skillet from the grill and set it down on a heat-proof surface.
Grill the crab pieces over direct high heat, with the lid open, for about 4 minutes, turning once, and then move them to the skillet. Return the skillet over direct high heat and gently turn the crab pieces with tongs to coat them with the sauce. Cook until the liquid boils and the crab is fully cooked, 2 to 3 minutes. Serve warm with pieces of bread to dip into the liquid remaining in the skillet.