Wont shift HELP!!!

[JuggaloDk17]

I have a '77 F-250 with a C6 and it wont shift out of 1st gear. I bought the truck after it sat for about a year and I have been doing work to it here and there and I took it for a ride up my street and it wouldnt come out of 1st. Anyone have any ideas where I should start looking for my problem? the tranny is full of fluid so I know I am good there. Please if anyone can help let me know.

 

[Spencer]

I bet its your modulator on the transmission, theres a vacuum line goin to it. Check that, make sure it aint leakin...itll be kind obvious should be the only vacuum line goin back there outta be on the same side as the dip stick. If it is in fact leakin replace the line and itll shift again. If not then you probably need a new modulator.