You might wanna try just opening it in the Jemplode animation editor as-is and seeing if it works. Maybe it'll just do the color reduction for you. It might not dither, but for a video like that, you might not need dithering and solid color posterization would work best anyway.

I personally don't know how to use ImageReady. It might not even have the option you're looking for... so many modern graphics tools are geared towards modern systems with unlimited color display capabilities. Palette manipulation tools are getting rarer and rarer.