In a large bowl, combine the pluots, jalapeno, pickles, EVOO, tarragon, lemon juice and 1/4 tsp. salt.
Preheat a grill or grill pan to medium-high. In a small saucepan, add the garlic, butter and 1/2 tsp. salt and cook over medium heat until the butter is melted. Brush the tops of the crabs with the garlic butter and grill buttered side down until the shell is bright red and charred in spots, about 2 minutes. Brush the undersides of the crabs with the garlic butter, flip and cook for 2 minutes more. Serve 2 crabs per bun, topped with the pluot relish.