It would be nice to know how much travel it should have. Maybe your friend can answer that....
About all you can do is see if there is any binding in the linkage. If it's all free down to where it enters the transmission or axle, that's about as much as you want to do. It's possible the linkage inside the differential is bound or locked up. Unless you know what you're doing, it's not advisable to go into the differential.
I'm sorry to say this, but it's probably getting close to taking this tractor in to a professional for repairs.