Just be aware that 763 does not have a dedicated/individual charge pump for the hydrostatic drive, it uses the return oil form the control valve & auxilariies to feed oil to hydrostat as charge pressure. Using the direct to tank return on the post driver by-passes that flow, so while the front auxilary is engaged and Hi-flow turned on to run the post driver the hydrostatic pump will have no charge pressure. Do not leave the front auxiliary turned on for extended length of time and do not try to drive the machine while it is on.
If you run the post driver with normal/standard flow (Hi-flow off) then the Hi-flow pump will provide charge pressure.