Abstract: The post driver apparatus of the instant invention is designed to be mounted on a truck chassis that can be readily moved from site to site and comprises a fluid-pressure impact hammer mounted in a shock absorbing cage assembly that permits rapid driving of posts into the ground without transmission of substantial shock forces to the frame and chassis on which the post driver is mounted. The apparatus is also equipped with means to vary the location and orientation of the impact hammer with respect to the truck chassis thereby minimizing the precision with which the truck chassis must be located.