A method of installing an axially adjustable driveshaft assembly in a drive train system is described. Two hollow driveshaft sections, where one has a valve through an aperture in it and each has an end fitting, engage each other to form an axially adjustable driveshaft assembly that has a sealed internal chamber within. When a pressurized air or vacuum source is connected to the valve, air fills the chamber or air is removed from the chamber, which correspondingly increases or decreases a drive train system length. This results in facilitating the installation of the driveshaft assembly between a transmission and an axle assembly in a drive train system.