1998 F250 shakes uncontrollably.!!!

My truck shakes so severely starting at 30
and beyond. If I try to gently throttle it past
30 at that point and from then on it will jar
you to death, yet if I were to get down on
the throttle the problem goes away until i
let up and preceecd to cruise at the
desired speed.  It idles fine, it purrs
between 0mph and 30mph, no spuddering
or weird gauge movements, the oil fluid
along with tranny fluid has been changed,
the tranny and torc converter have the
correct amount of fluid needed also with
no leaks, the drive shaft and tires are
balanced, I have changed the crank
senser, egr valve, air filter and cleaned the
mass air flow senor along with the throttle
body itself.  I have even changed the all
plugs and plug wires along with the throttle
positioning sensor.. I have also replaced all
vacuum hoses as recommended by
someone on another forum and still now
change. I also forgot to mention I have too
replaced the fuel filter and pump. PLEASE
HELP