it can be the pinch bolt at the top of the selector rod which sometimes comes loose but you dont need to remove the lid to do that,the main problem is usually the top needle roller bearing,if you look at the selector rod going into the top cover there is an oil seal there,just under that is a needle roller bearing which is bad for seizing,you may see a lot of brown rust deposit on top of the g/box cover,you cant get these bearings separate now and you have to take the complete housing which is very expensive but if you get the lid off and apply plenty of lubricant/oil/wd40 you can usually get it moving and freed off and in future give the shaft on top of the g/box a lubricate now and again which should prevent it happening again,good luck
cdc, November 2012
