SHAKY MOTOR... please help

ok i have a 72/74/79 ford f-250 highboy with a 351 clevland 4 speed 4x4. its got plenty of power but when u rev it up or even drive it, it begins to shake.. but when it just idles its fine .. smooth as silk.. i get up in the rpm's and it feels as though it wants to pop.. doesnt feel as though its moving very fast.. i made sure its in 2 wheel drive ...so i wouldnt blow it up.. i just bought this truck and really cant wait to restore it. let alone drive it.. but am too afraid its gonna ruin it they way it is.. any help would be great.. thanks... -Brett

When was the last tune up?

sounds like the engine mounts are on the way out. rubber mounts can rot away and eventually they can't isolate the engine vibration. almost all engines (except big diesels) are pretty smooth at idle, but vibrate more at high revs.

take a look at the engine mounts, if they look all worn and rotted, it's time to pull the engine and replace them. if it's not that, i'm not sure what it is.