In a small bowl, whisk together the brown sugar, ketchup, soy sauce, sherry (or chicken broth), ginger, and garlic. Set aside 1 1/3 cups of the mixture for basting, then cover and refrigerate.
Divide the remaining marinade between two large shallow dishes. Place 12 chicken thighs in each dish, turning them to coat well. Cover and refrigerate for 8 hours or overnight to marinate.
When ready to cook, drain the chicken and discard the marinade.
Preheat the grill to medium heat and oil the rack. Grill the chicken, covered, for 6-8 minutes on each side or until the internal temperature reaches 170°F. During the last 5 minutes of cooking, baste the chicken with the reserved marinade.