Well can't say you didn't search
http://www.blasterforum.com/threads/shifter-problems.46478/
Splines are the little teeth on shifter shaft and inside shift lever.
If you decide to replace the shift rod, clean area around rod by sprocket. clean
ALL dirt and rust off shift shaft
BEFORE pulling it through/out engine. This will help prevent damage to seal or engine cases or dirt inside engine. Maybe even coat shaft with grease or oil before pulling it.
I can't tell from pic, but if shifter was welded to shaft, cut shaft then file/grind burrs before pulling it.
Replacing shaft is relatively easy. Pull clutch cover basically. Might have to pull clutch? This is where a Clymers or download manual helps.