One more thing you may try before opening it up; Try putting a spring on the KD lever on the tranny so that it will be pulled all the way off. I have fought this issue myself when the return spring on the KD lever at the carb had broken and I didn't know it.

When the KD linkage wasn't being pulled all the way closed, my tranny did some really weird late upshift stuff. Some of it in 1-2 but most of it was in the 2-3.