Now having brought the truck home I finally had a decent check of the tyres and found that the inner ones are 15psi lower than the outer which are 55psi, this could explain the "violent shimmy" as said before.
Another thought has occurred, if the truck is not loaded would it need the 55psi in the tyres, I know in the Reo I used to get a bounce when empty so lowered the tyre pressures and it went away, would this do the same on the jimmy?