Combine chicken in a large zip bag with mango nectar, vinegar, olive oil and thyme, along with a generous sprinkle of salt and pepper. Zip the bag closed and move the chicken around in the bag to coat with the marinade. Lie the bag flat in the refrigerator for at least one hour and up to 2 hours, turning the bag every so often to redistribute the marinade.
Preheat the grill to medium high heat. Spray the grill with grilling spray (or rub the grates with oil).
Drain the marinade from the chicken and grill about 4 minutes per side, or until the chicken is cooked through.
Serve grilled chicken topped with chopped mango and thyme sprigs, if using.
Notes
Mango nectar can be found in your market's juice aisle. It comes in a can (same size as a soda can), and the most recognizable brand is Kerns.