if you look on top of the g/box where the selector shaft passes through a seal,they are bad for seizing on small needle roller bearings below that seal,if you soak the shaft with wd40 and keep working the gear lever back and forth and eventually it should free off,it may take a while but persevere,when freed off spray with grease/oil and every so often keep it lubricated and you shouldn`t have any problems again.If it doesn`t free off you can`t get the needle rollers separate now and you have to purchase the whole selector mechanism-very expensive so a scrappy would be worth a go.
cdc
January 2014