First, lubricate the linkage with WD40 or, I prefer, PB Blaster. Just follow the linkage under the tractor and spray the heck out of all of it. While you're under the body looking, have someone move the lever for you so you can trace the path.
It should be much easier to do afterward. Sometimes it's necessary to move the tractor either a little forward or backward also and then try changing. If you have a loader, and it still sticks some after spraying, lift the front just a bit and then change it.
If it's really too hard after that, have your dealer look at it as something is not right. John