Well, I recently attempted to fix my odometer because it was bugging me. This is out of a 72 fj40 BTW. The gears were completely stuck, so here's what I did. First thing was to get the odometer out. I took out the guage cluster (after disconnecting the battery off course)
To remove the odometer, I had to take off a small metal clip on the left side. This allowed me to move the bar over enough to slide the opposite end up and out. I didn't have to remove the speedo plate to get the odometer out. It just slid up and out. Once that was done I disassembled it completely to this.
Now there is something extremely important to watch out for during the disassembly. This small retaining clip lies near the white gear.
This little bugger will go flying when you remove it. I used two tiny screwdrivers to pry the thing off of the bar and lucky it ended up landed on me, otherwise I think I would have lost it.

