I have the same tractor and had the exact some thing happen to me this summer. I checked linkage, but everything looked okay from the shift lever down to where it went into the top of the housing. The block/section that the rod goes into is only held on by four bolts, but to get to them and to get it off, I had to remove the seat, toolbox, unhook the shift linkage, and remove the gray plactic cover under the seat. You have to remove the knob and rod that is used for controlling the speed of the 3pt hitch as well. After all that was off, you can remove the small top portion of the case where the shift rod enters. In there I found a small pin/shaft that fits into the the linkage shaft, then goes down thru a plate and to the shift forks. This pin/shaft was broken, but it was all there, nothing had fallen down in. The part was called ARM MAIN SHIFT and the part number was 6C040-18252. I think the part was about $20. I have the manual in pdf format so you can see what I am talking about. PM me if you would like me to send it to you. Let me know if you have any questions.