97 Chevy Silverado - shaking

I have a 97 Chevy Z71 Silverado. When I am going up a hill at say.........55 (or more) and I maintain that speed until I reach the top the truck shakes and acts like it does not want to take the gas.......I have to "Floor it" to get it to go and stop shaking. Any ideas? Could it be transmission or cracked heads? When I take it to my mechanic he can never get it to do it. If it is a very long hill the engine light will come on and stay.