Automatic transmission won't shift until you let off the gas?

Its 79 ford f250 automatic c6 transmission and 460 what happens is the truck won't shift to the next gear under normal acceleration unless you let of the gas, i don't know much about tranny, the kick down cable is hooked up. Any help is appreciated, thanks.

Sounds like a linkage problem. If you have wire cables look for fraying which could be gumming thing s up. There's a definite procedure for adjusting the linkage.

Sounds like a vacuum problem to me. I don't know s*() about Fords, though.

The vacuum modulator, mounted on the outside of the transmission, is not working. Follow the vacuum line from the engine to the transmission and it will lead you to the modulator. May just have a vacuum leak. Easy and cheap to replace if necessary.