Preheat a grill to medium. Toss the kielbasa, sweet potatoes, onion, celery, apricots and 4 thyme sprigs in a large bowl with 2 tablespoons olive oil. Drizzle with 1/4 cup orange juice and season with 1/2 teaspoon salt and a few grinds of pepper.
Tear off four 24-inch sheets of foil and fold each in half. Pile the kielbasa mixture on one side of each foil sheet. Fold the foil over and crimp the edges to seal well. Grill the packets, rotating them occasionally so they don't burn, until the vegetables are tender, 18 to 22 minutes.
Meanwhile, combine the vinegar with the remaining 1 cup orange juice, 1 tablespoon olive oil and 2 thyme sprigs in a medium skillet; season with 1/4 teaspoon salt. Bring to a boil and cook until syrupy and reduced to about 3 tablespoons, 5 to 7 minutes. Discard the thyme. Open the foil packets, drizzle with the sauce and sprinkle with celery leaves.