Drain pineapple slices, reserving 3 tablespoons juice. Set pineapple aside. Combine reserved juice and 1/3 cup jelly, stirring until smooth. Place chicken breast halves in a large heavy-duty, zip-top plastic bag; add jelly mixture. Seal bag, and shake until chicken is well coated. Remove chicken from jelly mixture. Pour jelly mixture into a small saucepan; bring to a boil. Remove from heat; set aside.
Coat grill rack with cooking spray; place on grill over medium-hot coals (350° to 400°). Place chicken on rack. Grill, covered, 5 minutes, basting once with reserved jelly mixture. Turn chicken, and place pineapple slices on grill with chicken; baste with remaining jelly mixture. Grill, covered, 6 minutes or until chicken is done and pineapple is thoroughly heated. Remove chicken and pineapple from grill. Cut chicken into thin slices.
Spread inside of each pita pocket with 1 tablespoon cream cheese and 1 teaspoon jelly; place 2 spinach leaves and 1 pineapple slice in each pocket. Arrange chicken evenly in pockets. Serve immediately.