Like was just said it is most likely the key from the driveshaft to the speedo drive right under your secondary clutch. 3 botls and 10 min to change. However, beware that the main cause of this happening is that the bearing on that end of the driveshaft is starting to go out and moving around on you. You do not want that bearing to fail in the back country. A $20 bearing can and will take out the chain and chaincase on the other side costing you big bucks and a miserable time trying to get it back to the truck. Sometimes you just get moisture in the speedo cable and it's frozen. That too can take out the little key, but if you haven't changed that driveshaft bearing in the past couple of years, now might be a good time to do it.