In a small bowl, whisk together lemon juice, garlic, basil, and mustard. In a slow steady stream, whisk in oil. Pour into a large resealable plastic bag. Add shrimp, tomatoes, red onion, and zucchini. Chill for 2 to 4 hours.
Place wooden skewers in enough water to cover; soak for at least 30 minutes.
Spray grill rack with nonflammable cooking spray. Preheat grill to medium-high heat (350° to 400°).