Long story short, it needs a new speedo head. If you are intent on actually repairing the speedo head, get some gears from here. http://www.instrumentclusters.com/odometer_gears.html I don't know ant other outfit that has parts for that head. It is just easier and a lot quicker to grab another head and swap it out.
The odo. on my F150 did the same thing a few weeks ago. I considered replaceing the broken gear but decided to just replace the whole gauge cluster. I am glad I went that route cause after the swap I took the old one apart to see what the trouble was and destroyed the old one actually getting to the broken part. In my case it was a broken worm gear. If you want to fix it yourself check out this site: http://www.odometergears.com/ Apparently this is a very common problem on Fords.
