Have you tried disconnecting the cables from the linkage on the gear box? If the lever is still stiff with the cables disconnected, then it's likely to be a problem with the cable.
The problem with the pin on the gear box linkage is actually the aluminium housing; it corrodes, crushing the thin nylon bushes onto the pin,making it difficult to rotate. The trick is to thoroughly clean out the part where the bushes fit in; you should be able to fit the bushes easily with your fingers. Then grease and refit the pin, ensuring it moves freely before refitting the cables.
I would advise against cleaning the pin with anything abrasive (unless it's already rusted) as this will remove the thin anti corrosion coating, allowing it to rust and cause problems in the future.