Submerge live lobsters in boiling water for 2 minutes. Remove from water. When cool enough to handle, remove heads from tails. With large knife, cut tails in half lengthwise. Place cut tails in large mixing bowl and toss with garlic, onion, thyme, olive oil and vinegar. Let marinate at room temperature for about 30 minutes.
Preheat a charcoal grill to medium-high heat. Once fire is hot, scatter fruit wood on top and allow to burn. As soon as the wood has turned to coal, place lobster tail halves on grill, meat side down. Cook until shell turns a lighter shade of red, about 3 to 5 minutes. Turn lobster tails and cook for an additional 3 minutes or until the meat begins to pull away from the shell.
Remove lobsters from grill. Place on platter and pour remaining marinade from bowl over finished tails.
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