Raise the vehicle and support it on jack stands, then remove the transfer case rock guard and drain the oil. Mark the position of the appropriate driveshaft flange in relation to the transfer case flange before removing the bolts, and then disconnect the driveshaft. Remove the lock nut from the transfer case companion flange, reinstall two of the driveshaft mounting bolts to prevent the flange from turning, and engage a pry bar between them. Use a puller to remove the companion flange, then carefully pry the oil seal out with a screwdriver, ensuring not to scratch or damage the seal bore in the case, using a block of wood for additional leverage if needed. Lubricate the lip of the new seal with gear oil and tap it into position with a socket or block of wood. Place the companion flange into position, install the lock nut, and tighten it to the specified torque. Reinstall the driveshaft, ensuring the flange marks made during removal are aligned, then refill the transfer case with oil. Finally, reinstall the rock guard and lower the vehicle to the ground.