Prepare a grill for indirect heat. If using a charcoal grill, build the hot coals on one side only. If using a gas grill, heat one side only to medium-high heat.
Pierce the sweet potatoes multiple times with a paring knife or a fork. Place the potatoes along the outer edges of the indirect heat side of the grill. Cover the grill and bake until the potatoes are completely tender, about 1 hour. (If the potatoes are ready before you are ready to serve them, you can wrap them in aluminum foil and they will stay hot for up to 1 hour.)
Meanwhile, make the dip: Cook the scallions over direct heat, turning once, until charred on both sides, 2 to 3 minutes. When cool enough to handle, finely slice and add to a bowl. Mix in the sour cream, parsley, Ranch Rub and celery seeds. Set aside until ready to serve.
To serve: Split the sweet potatoes down the middle with a knife and spoon some of the ranch sour cream over, letting it melt down into the potato. Season with additional salt and pepper if desired.
(Alternatively, you can bake the potatoes in a 400 degrees F oven for 1 hour.)
Combine all the ingredients in a medium bowl and mix well. Makes about 3/4 cup.