Code 56 is a quad driver modual problem.... Or in actuality its seeing a problem...
Shift solenoid A and B
TCC Apply Solenoid
EGR 1 & 2 solenoids
Being that code 56 is the only code you have, and you don't seem to have any driveability issues other then 3psi of boost, I have to agree with Willwren, and tell you to check the Boost Control Solenoid, and related Vacume lines...
My thought is that the problem will either be a bad Vacume line or Bad Boost Solenoid...
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