Well, it appears that WS FTP has a syncronize feature, and I found a guide at
http://www.ftpplanet.com/tutorial/synch/
Another option, especially given that you have more than one person playing with the files would be to use cvs, subversion or a similar version control system. That would depend on the platform that the server runs, and weather or not you're able to install software.
I also just stumbled upon FTPvc, which is available in demo form at
http://www.download.com/3000-2160-10059540.html
Hope that helps.