I have an Edson steering system on my S@!lboat. I went to an Edson presentation last winter at a boat show. The Edson tech guy said the cable sheath should be replaced every 7 years. Mine is likely original from 1985. It is stiff in the same way the OP has described. I can move the rudder by hand when disconnected but with cables connected, I cannot budge it. The cable sheaths have zerks mid way so I've been pumping in grease with no appreciable change. I may try squirting light oil at the top of the sheath. I have not looked into the top of the pedestal yet where the gear & sprocket are located. Lots to disassemble. Thanks for the ideas...