Place the potatoes in a large pot, cover with cold water and add the salt. Bring to a boil then continue to cook for 20 minutes or until fork tender. Drain the potatoes, and when cool enough to handle, cut into ¼-inch cubes.
Place the cut potatoes into a large bowl. Toss the potatoes with the jalapeño pickle juice, then gently stir in the scallions, dill, jalapeños, sour cream, mayonnaise, and black pepper until well combined. Taste and adjust seasonings, adding more salt if needed.
Cover and refrigerate for at least 1 hour, though preferably overnight, before serving.