26 tt truck, when going from low to high you have to play with the shifter to go in high gear. Everything looks good on the shifter under the truck. What needs adjusting
Might be most any type of obstruction that prevents the Clutch Adj. Screw from moving completely off the Clutch/Emergency Brake Lever Cam.
Sometimes the floorboard slot, because of shift of floorboard, restricts the lever from going all the way forward. That Clutch Adj. Screw (bolt) has to fall off the cam completely to engage the clutch for high speed.
Or could be some mechanical interference in the lever or pawl or toothed quadrant on the frame.
Then, you might have to do some re-set of "Free Neutral" if the lever won't co-operate in setting the clutch spring free to put the Ford into high.
Do you have an accessory transmission or maybe a 2 speed rear end that uses a "stick shift", or are you referring to the clutch pedal when you say "shifter"?