1. Heat grill to medium and oil the rack. Place corn, bell peppers, and zucchini on rack. Grill vegetables, brushing with 1 tablespoon oil and turning occasionally, until tender, 10 to 15 minutes (about 10 minutes for bell peppers and zucchini; 10 to 15 minutes for corn). As each vegetable is cooked, remove from grill and set aside (see note).
2. Stand ears of corn upright on a cutting board and, using a sharp knife, slice off kernels. Dice the bell peppers and zucchini. Place vegetables in a large bowl. Add onions and lemon juice. Pour remaining 1/4-cup oil into a small glass bowl; add garlic and salt. Microwave for 1 minute.
3. Thread shrimp onto skewers. Brush with 2 teaspoons garlic oil and season with salt and pepper. Cook for 1 to 2 minutes on each side, just until cooked through. Add balsamic vinegar to garlic oil and blend with a fork or wire whisk. Add 2 tablespoons dressing to corn salad along with shrimp and toss gently.
4. To serve, divide greens evenly among four plates. Top with shrimp and corn salad. Drizzle with remaining dressing.