best i can tell from the manual the GM engines use the woodward system and the mitsu 4Gxx engines use the juniper system.
there is also a Theta 2.4 LPG engine and a diesel engine

i guess we can rule out the diesel :o)

but in any case to get diagnostics you can either use the pc service tool or using the foot pedal. (depending on the fuel system that is on it)
The routine is similar to the mitsu/nissan pedal dance but not quite the same.
says to:
'turn on the key but do not start the engine. As soon as you turn the key to the on position, cycle the foot pedal by depressing it to the floor and then fully releasing it. You must fully cycle the foot pedal three times within five seconds to enable the code flashing feature of the ECU.'
- MIL flashes for each digit of the fault code, with a short pause (2 seconds) between digits and long pauses between different codes(6 seconds) Zeros are represented by a single 3 second long flash.
- starting code: 12
- trailing code: 12
every DTC contains 4 digits.
The code list will continue to repeat until the key is turned off.

Now to clear the codes when you are finished and the repairs are made do the same foot pedal routine except only press the pedal 10 times within 10 seconds.
- key on, press pedal 10 times within 10 seconds... codes should clear.
There is a note recommending to not use this clear mode as it erases any data stored in the ecu pertaining to faults that may be used for diagnostics on other code issues.
That's up to you though, if you are satisfied with the repairs and are confident that the truck is fault free then clear them.

And if i read the book right, if this system is a juniper fuel system only the pc service tool can be used to pull fault codes.

all i can say is give this a try and see if it works :o)