In a bowl, combine grapefruit juice, orange juice, garlic, oil, and juniper berries.
Pull most of the leaves from each rosemary stem, leaving about 2 inches at the tip of the sprig. Mince enough leaves to make 1 teaspoon; add to bowl. Reserve extra leaves for other uses.
Add shrimp to bowl and mix. Let stand at least 15 minutes, or cover and chill up to 2 hours.
Meanwhile, put rosemary stems in a bowl and cover with water.
Drain shrimp; discard marinade. Form 1 shrimp into a C and push thick end of a rosemary sprig through fat section, then through tail end of the shellfish. Continue skewering shrimp with remaining rosemary, distributing shrimp equally among sprigs.
Lay skewered shrimp on a barbecue grill over very hot coals or very high heat on a gas grill (you can hold your hand at grill level only 1 to 2 seconds); close lid on gas grill. Turn shrimp as needed to cook evenly until they are opaque but still moist-looking in center of thickest part (cut to test), about 4 minutes.