Update-
I tore the entire linkage out, from the handle to the lever on the side of the tranny, cleaned it up and lubed the linkages. Minimal improvement, so I disconnected the linkage from the shift lever on the tranny and checked the resistance there- very high.
I was considering pulling the assembly (the lever, a seal and presumably a bearing in an aluminium casing), so I removed the two bolts holding it in and was wiggling the mechanism to try and slide it out. Now it shifted easily, if the entire assembly was rotated. Obvious conclusion, the bearing or sleeve or whateverthehell in the assembly is tits up. About this time I decided that pulling parts out of an auto tranny without knowing exactly what's going to go "SPANG" off into the distance is a bad idea, so I bolted the bastard back up and called it a night.
So, does anyone know if this lever can be safely and easily removed and replaced, without cracking the case or other unpleasantness?