So I've had two clusters, 81 pickup all stock engine and drivetrain wise anyway. On both the odometer did not work, but the speedometer worked just fine. (Only replaced the old one because the needle had broken off in the speedometer)
now ive looked around on this forum, and it seems to be pretty common for both the odometer and speedo to stop working, or just the speedo, but haven't found any the other way around

 

I have not opened up a 1981 gauge cluster before, but if it is like the later trucks, you may have some broken plastic gears and/or stuck digits in the odometer assembly.

If the speedometer works, then the speedometer cable is not broken.

You will probably have to open up the assemblies and inspect for broken or stuck parts.

 

Yeah I was kinda thinking the same. Gonna grab the old one and crack it open so I don't break the one I actually use.