You can instantiate an object for the JSONUtil.cfc anywhere in your code that you want. I usually instantiate it in my Application.cfc onApplicationStart() method and store it as application.JSONUtil (as a variable in the application scope). I keep all my utility cfc's in a "cfc" folder off the web root, so to instantiate the CFC in my Application.cfc I have this:
application.JSONUtil = createObject("component","cfc.JSONUtil");
Then I can call application.JSONUtil.serializeJSON() or application.JSONUtil.deserializeJSON() wherever I need to in my code.
HTH,
-Carl V.