For 4WD trucks, the rear propeller shaft bearing is made of an unsuitable material, and therefore, when the vehicle is driven at a high speed for long hours, it may happen that the temperature of the lubricant becomes too hot and that copper sulfide is produced in abundance. If this problem is left unattended, the copper sulfide accumulates in the bearing and hinders the movement of the bearing, and this can cause the temperature of the lubricant to rise further. If this occurs, the bearing may come off the transfer case causing abnormal noises, and in the worst case, the transfer case and the output shaft could break, and as a result the rear propeller shaft come off causing the vehicle to stall.