i have had this problem before. have you checked the shift lever to see if it is bent? i was so scared the first time this happened, and it turned out to be the shifter being bent too far towards the engin that when pulled up it didn't clear the case. the stock levers are so soft all i did was get off and bend it back with one hand. if this does not work then i don't know what to do. i have never heard of any serious gearbox problems for these bikes before, so it may not be a big problem. just bend the lever out and it should work.
LOL! Same thing happened to me when I was out riding. Boy was I relieved to find the shifter had just bent and stuck against the case! I found a 24mm socket on an extension works great for bending it back out.
The OEM shifter is soft as butter. You may try an aftermarket unit if the problem continues.
hey, what happened to the MoJo? i was gonna give you some MX, but its not there. (yes, i have been under a rock for a while, the internet got screwd up LOL)