Prick sweet potatoes several times with fork; place on paper towel in microwave. Microwave on High 8 to 10 minutes, turning potatoes over once halfway through cooking, just until potatoes are almost tender. Let stand 10 minutes.
Heat gas or charcoal grill. Cut potatoes in half lengthwise. Scoop pulp into medium bowl, leaving a 1/4-inch layer in each half. Mash pulp well. Stir in 2 tablespoons butter, the marmalade and salt. Spoon mixture into potato shells.
When grill is heated, place potatoes on gas grill over medium heat or on charcoal grill over medium coals; close grill or cover with tent of foil. Cook 8 to 10 minutes or until thoroughly heated. Top with butter before serving.