I purchased my older tractor used and it had been in 4x4 for a while and did not want to disengage. I talked with lots of folks, before getting really involved with getting it out of 4x4. I sprayed all connection points of linkages including where it went into the case, I did not go inside the case, but that was one suggestion I received.
When I finally got it out of 4x4 mode, I had again soaked all linkage connections and then removed the knob on the lever and put a cheater on the shaft of the lever. Then with a steady pressure on the extended lever, I went through all the tricks I had tried—lifting front wheels with loader, driving in reverse, turning front end while driving and others. It took a bit, but it finally moved and then came out. It has never really been what I would call smooth shifting between 2x4 & 4x4, but with a steady strong pressure, it shifts between since. I also try to be sure to stay shifted out of 4x4 until it is needed. I will say that this tractor had less ability in 2x4 than the previous 2x4 only tractors. It was purchased for the loader and not the 4x4, although 4x4 was recommended when looking for a tractor with a loader.