Tractor Repair: IH 684 stuck in low gear, shift linkage, diesel tractor

Question
I have an IH 684 diesel tractor that is stuck in low gear.  You can try and put it in neutral and when the clutch pedal is released it act like the tractor wants to go, but it won't. Prior to this I was moving hay and each time I took it out of reverse to put it in low it was taking more force to shift into low until it stuck in that position.  Is this a common problem and is this easy to fix.

Answer
  No it's not a common problem, but it has happened before.  There might be a broken shift fork or shift rail inside the transmission.  Try to move the external shift linkage on the side of the transmission that is operated by the shift lever to get it back to neutral.  If it's not possible, then the transmission top cover will have to come off to fix it.