50 Dodge Coronet  Gyromatic shifts up and down as it should in low range from first to second and back without use of the clutch, but in high range seems to be stuck in 4th gear. I have drained and refilled the oil (10w) three times and engine rpm is 450 at idle. I have a manual, and vintage chrysler service videos . I rebuilt the motor and carb , wiring, cooling system. Any help appreciated before I pull the tranny.