Put chicken, juice, Splenda, soy sauce, hot sauce, pepper and garlic powder in a zip-lock bag, seal, and shake to evenly coat the chicken with the marinade. Refrigerate for 30 minutes.
Remove the chicken from the marinade. Measure 2 tablespoons of marinade into a small saucepan and set aside. Grill or broil the chicken thighs for 4-5 minutes per side, until juices run clear. Transfer the chicken to a serving plate and set aside while you prepare the sauce.
Mix the cornstarch and water in a small bowl until the cornstarch is dissolved. Add the orange marmalade and rice vinegar to the marinade in the small saucepan and heat over medium-high heat until mixture is boiling. Reduce heat to medium and simmer for 1 minute. Add the cornstarch mixture and continue to cook, stirring constantly, until the sauce is thickened. Remove from heat and serve the sauce drizzled over the chicken thighs.