LLY going into limp mode need help

[bigdaddy9185]

I have been helping a friend with his LLY duramax in a 2004 5500 gmc topkick. The problem we are having is that the diagnosis per the diagnostic tool is low rail pressure. but here are the kickers to this ,the engine runs great without any load , but under a load it runs normal up to around 55 mph then kicks into limp mode and falls on its face
1) replaced #2 and 8 injectors after balance test showed that they were bad
2) repaired injector wiring harness on #2
3) repaired fuel line from tank to ficm
4) replaced fuel filter
5) replaced fuel rail bypass valve with blockoff unit
6) ran check on scan tool again still same problem
7) Replace fuel injection pump with another that came off of a topkick that was upgraded to a high performance pump , still no improvement same conditions
8) replaced map sensor
9) NEED LOTS OF HELP PLEASE AT A LOSS

I am wondering if the problem is or could be any of these listed items and if it is how to test them to be sure FPR, MAF, or ECM and or FICM

[bigdaddy9185]

also have checked the wiring in the harness for any frays or chafing and is all good

[Dashman]

Did you modify the input voltage vs. pressure table? Have you scanned with a chart for desired pressure and actual pressure?

[bigdaddy9185]

this was not tuned been trying to help him get back to texas so i can hook up and help , new problem today #2 injector leaking fuel again just replaced that injector. looking for help with where problems are coming from just to get him back so we can really get to the bottom of problems

[Dashman]

what about the pressure regulator and the pressure relief valve?

[bigdaddy9185]

there is no fluctuation in the idle like FPR is noted for, and already replaced FPRV with a FPRV block off so I feel that those are good but it is giving symptoms of fuel starvation under load because it runs great when not loaded at all throttle levels and speeds up and downhill just wont run under load

[bigdaddy9185]

and now the #2 injector is leaking again nothing lose and new lines and seals ??????? truck has a dam ghost in it