1. In a large bowl, mix together garlic, pineapple juice, sugar, ginger, peanut butter and vinegar. Set aside one-fourth of the mixture, then marinate the chicken cubes in remaining three-fourths of the ginger mixture for about 30 minutes.
2. Thread marinated chicken onto skewers. Heat a grill, grill pan or broiler on high heat. Cook skewers, turning occasionally, about 8 minutes, or just until chicken is cooked through. Remove from heat.
3. Meanwhile, toss sliced cucumber with reserved ginger mixture. Place a portion of cucumber on each of 4 plates. Place chicken on top and serve with tomatoes, lime wedges, cilantro and crushed red pepper if desired.
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