The side plate can leak from either the gasket or the seal for the output shaft. As Roland said, there is a 50/50 chance for pulling the side plate without pulling the ZF.
Most of the vendors have the gasket and output shaft seal. I know Wilkinson has them.
If it is the gasket that's leaking, there is a remote chance you can seal it without pulling side plate. Get a good UV flashlight and find where the oil is coming out. If it is coming through one of the nuts/washers, remove them and clean the area with brake cleaner. Then apply black silicon to the area and reinstall the nut/washer. Here is the light that I use.
Valvoline gear oil is visible with this light without adding dye. Other gear oils are probably visible as well with this light.
