The trip meter wheels and odometer wheels are usually seperate mechanisms but may be driven off of a common drive gear. If that is the case, the drive it probably OK since one works. The odometer drive may have a loose worm or gear or a broken tooth in the mechanism causing the malfunction. It will have to be opened up to repair and you may need a replacement part or two.
I'm more familiar with the Stewart Warner commercial units so I can't help with it much. A speedo shop with early Ford unit experience could help you more.
Kerby