Step 1: Rinse the shrimp under cold running water and then drain and blot them dry with paper towels. Peel and devein the shrimp. Place in a large resealable plastic bag.
Step 2: Place the cilantro, jalapeños, scallions, chopped garlic, salt, black pepper, and cumin in a blender or food processor and finely chop. With the machine running, add the olive oil and lime juice through the feed tube and purée to a bright green paste. Pour the marinade over the shrimp and let them marinate in the refrigerator, covered, for 30 minutes, turning the bag once or twice so they marinate evenly. (If not using a vegetable grill basket to grill the shrimp, thread them on bamboo or metal skewers.)
Step 3: Set up the grill for direct grilling and preheat to medium-high.
Step 4: When ready to cook, brush and oil the grill grate. Drain the marinade from the shrimp and discard the marinade. Place the shrimp in a single layer in the grill basket—don’t crowd—and grill until just cooked through, 2 to 3 minutes per side, turning the shrimp with a spatula or wooden spoon. Work in batches if necessary. (Alternatively, place the skewers on the grill grate and grill 2 to 3 minutes per side.) When done, the shrimp will turn pinkish white and feel firm to the touch. Immediately transfer the grilled shrimp to a platter or bowl.
Step 5: To assemble a taco, put several shrimp in a warmed tortilla. Top with shredded cabbage and a dollop of sour cream. Serve with lime wedges.
Try a wire mesh basket for grilling the shrimp.