Sprinkle shrimp evenly with chili powder. Remove husks from corn; scrub silks from corn. Place corn on a grill rack coated with cooking spray. Grill 12 minutes, turning occasionally. Add shrimp to grill rack after 6 minutes; grill 3 minutes on each side. Cut kernels from ears of corn. Discard cobs.
Combine shrimp, corn, lettuce, and tomato in a large bowl; drizzle with Lime-Cumin Dressing, and toss well. Add avocado; toss gently. Serve immediately.
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