1. Prepare grill to medium-high heat.
2. Sprinkle chicken evenly with 1/4 teaspoon salt and 1/4 teaspoon pepper. Place chicken on grill rack coated with cooking spray; grill 10 minutes or until done, turning after 6 minutes. Remove from grill; let stand 5 minutes. Cut chicken into 1/2-inch pieces; keep warm.
3. Cook pasta according to package directions, omitting salt and fat. Drain in colander over a bowl, reserving 1/4 cup cooking liquid. Place pasta in large bowl.
4. Heat butter in a medium saucepan over medium heat. Add garlic to pan; cook 1 minute, stirring occasionally. Combine 1/2 cup milk and flour in a small bowl, stirring with a whisk. Add milk mixture to pan, stirring constantly with a whisk. Stir in pesto. Gradually add remaining 1 cup milk and half-and-half, stirring constantly with a whisk. Cook 8 minutes or until sauce thickens, stirring frequently. Add 1/4 cup reserved cooking liquid, remaining 3/4 teaspoon salt, remaining 1/2 teaspoon pepper, and 1 cup cheese; stir until cheese melts.
5. Add chicken and sauce to pasta, tossing well to coat. Add tomatoes and basil; toss gently. Sprinkle with remaining 1 cup cheese. Serve immediately.