The Cummins zero launch feature allows one to release the clutch with no throttle input. The computer in the truck automatically adds fuel to the engine and away you go! I can take off on steep hills with my 16k fiver with no throttle input whatsoever.
I suspect your truck may not have this feature, but given the fact that your truck has the Cummins it will have plenty of low end torque. When pulling out of a boat launch, put the truck in 4x4 low as you indicate you have done, put the truck in first gear while holding your foot on the brake and slowly release the clutch until you feel it grab (will be noticeable by the engine starting to lug down slightly), slowly release the brake pedal and then fully release the clutch. This should allow the truck to start moving forward with absolutely no rollback at all. I have owned 3 diesel trucks with manual transmissions and I have found them all quite easy to take off with. The worst was my 6.0L Ford mostly because they have lousy low end torque.
I have driven manual equipped trucks for so long I honestly don't even really think about what I'm doing a lot of the time; it's branded into me!
I hope this helps. If you have any more questions fire away! :B