It sounds to me like the governor might be stuck, Depending on the year - I believe it is located under the tailshaft housing. Drop the driveshaft, remove the mount fasteners and then remove the bolts from the tailshaft housing, You will need to remove the speedo drive connection cable, but you may or may not need to remove the speedo drive from the housing. slide the housing off and inspect the governor for movement, it works on centrifugal force, so you can pull the weights out ( carefully) and feel if it does move or is stuck hard. Roger Just had his trans worked on as it had some "stuck" components from sitting. The C-4 is a pretty simple unit, the shifts happen when the governor moves, the vaccum modulator can modify the shift points but the governor is the main operator. Hopefully the gasket doesn't break when removing the housing, and be careful to not overtighten the bolts when re-installing. Don't be a gorilla, you can damage the case! HTH...