my taco wont shift on hills

ive got a 95 with the 4 cyl. in it. Its automatic. When you get to a hill in this truck it will lose speed as u start climbing,(roads by the way) an it wont downshift, like it dont know what gear to be in, you can hold it to the floor and its no better, and then eventually it will downshift and the rpms jump to above 4000 or more, they scream. But when you get back on flat roads it shifts fine. any idea whats causing this?

 

that and check you kick down cable make sure it has some tension on it