Place chicken pieces in large slow-cooker.
Mix together zest and orange juice, soy sauce, garlic, honey, brown sugar, salt and pepper. Pour over chicken pieces.
Cover and cook on high for 3 hours or on low for 6 hours.
Pre-heat broiler to high. Place rack in upper 1/3 of oven. Remove birds from slow-cooker. Strain juices into a sauce pan.
Over medium heat, cook juice until reduced by about half, and slightly thicker, about 10 minutes. While sauce is reducing, Place chicken pieces on a foil lined baking sheet and brown under broiler about 5 minutes, or until skin is golden brown.
When sauce is reduced, add Grand Marnier and whisk in butter, 1 Tablespoon at a time until sauce is smooth.
Place chicken on a platter and pour sauce over chicken or serve sauce on the side.