All the major exhaust brake vendors have models for the Ford, e.g. PAC, BD, etc. They are all good. US Gear, Banks and others also offer them but I haven't heard any reports on those.
There is also a less expensive kit that converts the Ford's warm-up butterfly valve in the turbo bypass to an exhaust brake. Does not give as much engine braking as a BD or PAC but does a pretty decent job. One such kit is the Bully Dog brake - see http://www.thepartsbin.com/sitemap/ford~exhaust_brake~parts.html Another such kit is from SP Diesel - http://www.spdiesel.com/