I've never heard of such a thing before. On rangefinders, there is a small focusing mirror behind the small window next to the viewfinder. This mirror is on a vertical pivot that is twisted when you focus. Usually, it is the lateral movement that gets affected (and throws focus off). But this happens when the focusing mechanism gets loose.
If there is a loose part, it should make some noise when you tilt the camera...
Polaroids are notoriously difficult to take apart. There might be a small adjustment screw inside for the vertical alignment, I really don't know. But keep in mind that taking the viewfinder assembly off might very well wreck the camera.
I checked my Polaroid 315 (with head size focusing jig) and it has two screws behind the viewfinder. I suspect these to be holding the front panel. If you really want to be adventurous, you can try and look inside. But I would be somewhat scared to do so since my camera repair skills are not very good... If it was mine, I would try and live with it even if it would bother me to no end...
Don't forget that if it works, don't fix it.