Some possibles are:
rust encrustation (yes, stainless can rust in some circumstances)
crushed outer rudder
bush or swollen fibreglass (water intrusion) pushing on post
fine line (fishing?) wrapped around rudder
a thousand other things that have stuck between the post and bush.
Have you checked to see whether it's both rudders?
If you first get to the boat when it's on the hard
, put your ear to the rudder when it's being moved. A crunching or scraping sound would indicate encrustation.
It shouldn't be too had to drop the rudders and check for cleanness and straightness. Someone in the yard might be able to do that before you arrive.