I was thinking of just swapping the whole cab also. Since it is only held on by a few bolts that would make all the diesel stuff go on the 4x4 frame along with the engine. Both trucks are automatics so I think the "diesel" trans should plug into the transfer case fairly easily, and we just had it rebuilt last year.
I did the same scenario body swap (2x4 to 4x4) just like this with two early '70's Ford 250 years ago and it went very easy. My biggest concern with the Ram's is/was mounts. I have not ever had to pull a Cummins yet so not familier with the mounting systems. I know some Fords you have to cut out frame mounts when switching engine models.
Also I have heard that the gas rear differentials can be a smaller size Dana, but again that is easy to swap and use mine. It was rebuilt a few years ago too (had won a ATS cover at May Madness but it cost over 1000 to have it installed because the diff needed new gears, bearings, clutch packs, seals etc. SO much for FREE hahaha) Not sure if the front diffs are the same between gas and diesel?