If you have a good fuel filter , then that leaves the injection pump and the injectors .
Do you have the VIN number? I can check to see if injectors have been done under warranty , If you are a second owner of this truck. If you are the original owner, and are positive no injectors have been replaced, then this is most likely your cause of the P0093 low fuel rail pressure.
Here is a part from the special policy on injectors for the duramax, as to what there are replaced for.
Some customers of 2001-04 model year Chevrolet Silverado; GMC Sierra; 2003 Chevrolet Kodiak and GMC TopKick vehicles, equipped with a 6.6L Duramax Diesel (RPO LB7 - VIN Code 1) engine, may experience vehicle service engine soon (SES) light illumination, low engine power, hard start, and/or fuel in crankcase, requiring injector replacement, as a result of high fuel return rates due to fuel injector body cracks, ball seat erosion, or high pressure seal extrusion.
They were only covered for 7 yrs or 200000 miles.
Diagnostics with a scan tool to read the balance rates, and fuel pressure readings would be needed to verify. I would also graph( with scan tool) the fuel pressure regulator to see if it is sticking. A conditon of this would be a surge at idle when warm.
After the injectors are replaced , then the injection pump can be tested for max fuel pressure reading. It should read at least 155-160 MPA. If it still cannot reach this pressure then the injection pump will also need replaced.
( the pump will not be able to reach full fuel pressure if the injectors are faulty. Injectors will need to be checked and replaced first , then check the injector pump)
Both are expensive, so proper testing should be performed.