I'm in Green Bay and just getting into this also. They are working on getting an off road park in Forest county but that is a way out. Check out Legendary Civilian Jeepers http://www.lcjs.org/content.php as well as WOHVA http://wohva.com/ for info.
The forest roads in the Nicolet forest are nice but not extremely challenging. There are not any public wheeling places around Appleton that I know of.