With Hi-Low and the PTO lever on the cover there wlll be valves built into the cover. The tubes to these valves come in from the bottom, each will have an O-ring. There will be a retainer plate on the bottom of the cover it will have holes for the tubes with grooves cut for the O-rings.
So you will need a cover gasket and O-ring for each tube. If your shift boots are bad replace them too, major cause of water in the trans/hydrualic fluid.