As said check the kickdown linkage. But I usually find if the 1-2 shift is still normal it usually is not the throttle pressure kickdown linkage. We did have problems on some years as I cant remember which years where we would replace the front drum and piston and seals with updated parts for a late or no 2-3 shift. And I found reverse usually would still work normal since it uses much higher pressure. And I also some some with no codes where the gov pressure sensor and solenoid fixed it as we did have a lot of problems with them. Couse we would watch the gov pressure on the scanner to see what it was doing. Good luck with it, Ron