In a large airtight plastic bag or shallow dish, combine the sirloin, whiskey, and balsamic vinegar. Massage into the fibers of the meat. Place in the refrigerator for 1-2 hour(s).
Remove the sirloin from the refrigerator and allow to sit for 30 minutes. Meanwhile, heat the grill.
Remove the sirloin from the marinade and season with the salt and pepper. Discard excess marinade.
Preheat the grill to high heat (375°F). Sear the steak over indirect heat. If you are using a gas grill, cover the grill. Grill for 4-6 minutes on each side for medium rare. If you do not have a grill, you can follow the same instructions using the broiler of your oven on the second to top rack. Make sure to keep the door cracked and properly ventilate to not set off your smoke detectors.
Remove the steak and allow the meat rest for 5 minutes. Slice against the grain and serve.
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