Ranger TM (2006) shift problem

My Ranger (TaskMaster-2006) is now 10 years old. Me and my wife use it daily for hauling horse manure and any thing that needs to be hauled or moved on our farm.

The gear shift lever has recently started to lock up or get real hard to shift after the engine runs for a couple of minutes. It works perfect before it has ran for a few moments. Any experience of solving this problem is greatly appreciated. I am a complete novice to mechanics and only do the basic maintenance required.