This is set when the Hi/Lo clutches are put together. The wet clutches may need to be adjusted do to wear or inappropriate settings at time of assemble. The pressure on each clutch (Hi/Lo) is ajusted individually by shims. If it pops out of Lo gear or shifts extremely hard add shims. If it shifts to easy or slips take shims out. The same goes for Hi except there is an external bolt that needs to be adjusted, so the lever doesn't go to far forward in Hi.
I set mine so when the tractor is warm, it doesn't creap ahead with the Hi/Lo in neutral while the main transmission in 1st gear and yet doesn't slip under full load in Hi or Lo.
If your D19 is hard to put in Hi or Lo, it probably creeps with the Hi/Lo in neutral.
I would also recommend changing the hydralic oil and filter in timely manor. The hydralic oil is also use for the wet clutches in the Hi/Lo transmission or power director.