Random questions: Do you have a functional neutral? Is the tractor under a load with no movement when you let the clutch out or is it stuck in one of the ranges?
Without knowing your answers, here are my thoughts:
Before I went to pull the transmission apart, I'd take another look at the linkage. Over the years it can get sloppy, and getting things funky happens easily. Were you shifting from high to reverse? Sometimes the range shifter will disengage from the high range shift rod before it is actually fully disengaged. It may be hard to tell from the breakdowns, but there isn't much that can really go wrong with the range transmission. Low and Reverse are engaged by sliding a gear on a shaft. The gear is either engaged with a gear toward the front or the rear. This gear has to move a fair amount from low->neutral->reverse, and the gear isn’t that wide. That being the case, it probably isn't stuck in low and reverse at the same time. High is achieved by sliding a collar forward so that two shafts are coupled together. High and low/reverse use two different shift forks. If you look at the linkage, the high range arm is the forward most one. Disconnect the linkage and try to tap the arm rearward. This should disengage the collar. It won't move much from engaged to disengaged, but I'm betting that is where your problem lies. Once free, I would take a look at the bushings on the shifter. They are easy enough to replace if worn, and your friendly CaseIH dealer sells them cheaply enough. Also take a look at the pins used within the shifter mechanism. The bushings are the most common problem area.
BTW, if you must take a peek inside the range trans, the easiest way to have a look around is to remove the MCV on the left side of the rear housing. Once removed, you can see both the sliding gear and the collar. That probably isn't the best way to access things to repair it, but doing that will only require draining some fluid and a gasket/o-ring.