Raise the vehicle and support it securely on jack stands, then remove the rear wheels and brake drums. Mark the driveshaft and axle pinion flange for ease of realignment during reassembly, and remove the driveshaft. Install a torque wrench to the pinion nut and record the torque through several revolutions. Mark the relationship between the pinion and flange, hold the flange to keep it from turning, and remove the pinion nut along with the differential flange. Pry out the old seal using a slide hammer or Ford tool T65L-4851-A, then clean the oil seal mounting surface. Tap the new seal into place, ensuring it is inserted squarely. Inspect the splines on the pinion shaft for burrs and nicks, removing any rough areas with a crocus cloth and wiping the splines clean. Install the differential flange, aligning it with the marks made during removal, and tighten the pinion nut while allowing some movement for proper seating. Take frequent pinion bearing preload measurements until the original readings are obtained, then reinstall the driveshaft, brake drums, and wheels. Check the differential oil level and fill as necessary, lower the car, and take a short test drive to check for leaks.