Gently stir the pineapple chunks in juice, Italian-style salad dressing, chicken, shrimp, green bell pepper, red bell pepper, sweet onion, mushrooms, and garlic together in a large bowl. Cover the bowl with plastic wrap; allow to marinate in refrigerator for 8 hours or overnight.
Preheat an outdoor grill for medium heat, and lightly oil the grate.
Drain the mixture in a colander over a bowl to catch the liquid; shake to remove excess liquid. Alternately thread the chicken, red bell pepper, onion, green bell pepper, onion, pineapple, mushroom, and shrimp onto a skewer until each skewer is full; repeat until all ingredients are used.
Pour the strained marinade into a small saucepan and bring to a boil over high heat. Reduce heat to medium-low, and simmer for 10 minutes; set aside.
Cook the skewers on the preheated grill, turning frequently, until the chicken is no longer pink in the center and the juices run clear, about 15 minutes. Baste skewers generously with the reserved marinade as they cook.