not sure what vaporizer you have on this truck, i will assume it is an aisan? These usually operate at a pressure of around 32-36kpa (4.6 - 5.2 psi)
so the pressure you said is around what it should be.
your problem may be in the injector body or throttle body.

but as was suggested, if you are not familiar with this truck get a qualified service tech to come out and properly diagnose this issue.

good luck