Add the Dr Pepper, BBQ sauce, garlic cloves, kosher salt, and the crushed red pepper to a medium bowl. Stir well to combine the ingredients.
Slice the flank steak into long strips, about 1 inch wide. If the flank steak is very thick, cut it into strips, and then slice each strip in half lengthwise, to make two equal, long pieces.
Add the steak to a large, re-sealable bag. Pour the Dr Pepper marinade over the meat and reseal the bag. Massage the meat gently; ensuring the entire steak is coated with the marinade.
Refrigerate the steak for at least 2-3 hours, turning the bag occasionally so that both sides of the meat are equally saturated with the marinade.
Remove the steak from the refrigerator and let it sit out for just a few minutes to take the chill off prior to cooking.
Remove the steaks from the bag and discard the marinade. Thread the steak strips onto the skewers.
Spray the BBQ grates with a grill spray to prevent sticking.
Grill the steak skewers over moderate heat until the meat is charred in spots, and the steak reaches the desired doneness – about 5-7 minutes per side, depending on the thickness of the meat.
Arrange the skewers on a platter and serve.