for a gmt800 the (pre-2010) 9.25ifs doesnt swap easily into a 1500 iirc. the clamshell style case is completely different than a 2011+ that i swapped into my gmt900 (this wouldnt swap either) a long time ago a company used to make brackets to put the clamshell style ifs into a 1500 but it was 100% directed at lifted and offroading and accommodated a lift as well so it wouldnt be good anyway. i would just run what you have till it breaks - some never have problems.
the only traction aide i could find that works in a 9.25 is a ratcheting style locker that replaces the spider gears for my year front diff. for the older diff i dont think i ever actually found someone who's done it but supposedly theres a eaton elocker that "should" work but its 1200$+ if you can even find one.
4l80 is a good idea, pretty much any transfer case from a 2500 or 6l80 1500 will work (33spline input) all the transmission adapters use the same 6bolt pattern. i say pretty much because newer 6l90 4x4s use a 36spline input i think. i imagine you could keep make an electronic shifted trans work from a 2500 in a 1500 might take moving some pins around but its doing the same action i dont see why it couldnt work. i dont know which is best cause i dont plan on using them.
rear end , having a 3rd member style would be nice for changing gears but regearing the front diff repeatly would suck lol or just collect/build a front diff in each ratio. and the whole rear would have to be fab'd. the 14bolt 9.5 SF bolts right in with zero fab work.
other "problem" with 9.25ifs is gearing - currently have 4.10s in mine and im about out of gear going thru the traps @140+mph and 6500rpm+. longest ratio is a 3.42 which would get me to about 170 in 3rd gear (1:1 in a 4l80) which "should" be unobtainable but if i wanted to do a 1/2 mile even or something good chance it would break that barrier.
