In the Firefly series it was an ATV.
I know I've seen a thread SOMEWHERE discussing the make and model.
It is also discussed on page 29 of Volume Two of the Official Firefly Companion book.
The Mule licence plate is in a private collection and may be on QMx's possible-maybe-future projects list.
In the meanwhile, a replica of the Mule plate has been offered by this seller on ebay: http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=180490479902
The second Mule was the hovercraft in the movie, "Serenity".
If the ground mule is a Polaris I have a headstart. I have a Polaris 500. I guess that I will need the same info as Urbanmx to convert mine over. For some reason I don't think that the color red will work.
There was a great thread about this on Propcircle, but the site is gone. I managed to find the exact make and model, but with Propcircle gone I could only guess what it was. If memory serves, it was a 2001-2002 model Polaris.