Two ways to go:
You can drain the oil out through the back cover getting most all of the oil out, as the filter just holds a pint.
Then roll the differential/hydro assy out the back.
If you are not going to change the hydro oil, just roll the differential/hydro out and stand it up positioning it on the rear cover sitting it on a block of wood or brick whatever you have, then remove the hydro and replace the gasket, and reassemble.
Its much cleaner to have it sitting in one of those large shallow drain pans like they sell at harbor freight catching the fluid that leaks out when you separate the hydro from the diff housing, as you just cant prevent that, even if you drain everything.
