I have a 1995 Ford F250 4x4 with a 460. Between 1100RPM and about 1400RPM the whole truck shakes. It does this in park, power braking, and driving. I have replaced plugs, wires, distributor, rotor, and ignition coil to no avail. I think #5 cylinder might be missing. I tried swapping fuel injectors 5 & 6 and #5 cylinder still seemed to be missing. I am determining this by unplugging wires one at a time. #5 seems to be the only one that doesn't effect the engine when disconnected. I plan to check compression this weekend, but doubt that will show anything. Can anyone help with this? I am just not sure what to try next.

 

Try disconnecting the vacuum hose from the MAP Sensor and see if the vibration goes away or lessons. By removing the hose you richen the mixture (the computer thinks the engine's under load)