Heat grill to medium-high. In a small bowl, whisk together the sour cream, mayonnaise, milk, and cumin. Set aside.
Skewer the shrimp. (If using wood skewers, soak them in water for 10 minutes beforehand.) In a small bowl, combine the butter and garlic. Brush the shrimp with the garlic butter.
Place the shrimp on grill with the limes. Cook for about 4 minutes on each side or until the shrimp are opaque and the limes are browned.
Remove from grill. Lightly salt the shrimp.
Grill the tortillas for 30 seconds on each side, then place inside a paper bag to keep warm.
Pull the shrimp off the skewers; divide evenly among the tortillas. Top with the cabbage, sour cream sauce, tomatillo salsa, and a squeeze of grilled lime.