StealthDiesel

Ending up with the 2005 truck complete and running, regardless of what trans is in it, would make it worth much more than if you sell it as a roller or sell it for parts.

Time has passed 4 speed automatic transmissions. Once you have towed with a 6 speed auto trans, a 4 speed auto trans will drive you nuts.
I too would toss the auto trans in the 2003 truck.
My NV5600 6 speed manual trans is tiring at times but way better than the 48RE 4 speed auto.

Swapping in a 2wd G56 manual into the 2003 4wd auto trans truck is straightforward and has been done a ton of times.
Swapping in the 2003 4wd auto trans into the 2005 2wd truck, would require a ton of wiring harness changes to make it all work.
Plus I'm sure the 4wd auto trans would need a tail housing swap to work without a transfer case in the 2wd truck.

There is no simple solution that I can see for swapping parts around in these 2 trucks.

Southern welder

Thanks for the replies!
im not worried about the 05, it’s going to sit in storage and eventually behind my house as a parts truck, unlikely to see action again, it’s a 2x4 so not worth more then the engine and transmission IMO
im going to end up with a 2003 2500 on the road with a 6 speed manual 4x4
im just trying to confirm my wiring comparability, some say it’s easy and other say it’s not
i need to find engine and dash diagrams for both trucks

StealthDiesel

If you are not worried about keeping the 2005 2wd truck intact after the swap, I will go back to my earlier recommendation.

Swap the 2005 2wd engine and G56 manual trans into the 2003 auto trans 4wd truck. May have to install a different transfer case. Likely will have to modify both driveshaft lengths.
Flash in a custom program into the 2003 ECM to run the 2005 engine. The custom program will have to be manual trans and matched to the 2005 pistons and injectors.

A custom program using an HPT programmer is what Green machine did (posted above) plus his thread on it. A Smarty Touch will also work.

Southern welder

The deed is mostly done, just trying to figure out of the air bag system has shut down my fuel pump and gauge because one plug is not plugged into the air bag controller