I have been down that road before. It will be difficult to tell what it is without some real investigation. I would start by disconnecting the drive cable and attempt to shift the drive ( with the engine running ) by pulling / pushing the cable. It is possible the cable just stretched a bit and needs to be adjusted. Merc makes a tool that sets the cable where it is supposed to be, but it requires having the drive off. I had success with just playing around with the adjuster until the drive shifts smoothly. if it doesnt shift by manually pulling the cable, then I would take the rear cap off and take a look at the shift fork and all related pieces in there. If it all looks good, you will have to tear down the upper and check out the cone clutch, springs, etc..