This is a replacement Traxxas X-Maxx/XRT Differential Gear Set. This set includes two output gears, four spider gears and two thin diff washers. This set is only good for one differential; front or rear.
When reassembling the differential, be sure to fill it with differential fluid. From the factory, Traxxas filled the diffs with 10,000W in the front and 30,000W in the rear.