Here is what you're looking for.
http://www.thor4x4.com/I think they wait until they get enough orders to do a production run, so it may be a while before you can actually get them.
Slickrock gear used to make them, but they went out of business.
This is something I'd like to do someday, but it is so far down on the list, I don't know if I'll ever get there.