I
rebuilt my transfer case (link) by leaving in the trans.
If I had to do it again, I would do it the same way.
Removing the Tcase and getting it back on under the truck was doable, if not a little annoying. But I did this all by my self, no help lifting anything, no tranny jacks, no slings, just muscle power and a bag of swear words- all out on the dirt driveway lying on my back with my truck parked on 4 big logs

Alternatively, when
rebuilding it, I ended up replacing my entire gear set, and I had a donor tcase attatched to 5 speed trans- pulling it apart- breaking the staked nuts etc, was a lot harder to do with it off the truck than on (nothing to resist against) (one could of course make sure to do this before removing them). I needed help to lift and move the Tcase/Tranny assembly, and help at times to break it down.
I made sure to take photos of every single step, every time I removed something. It helped immensely.
Some of the photos I uploaded to a photobucket album, which you can view
here.
Alternatively, for the wealth of great photos and info, I found Hugh's rebuild thread to be of the greatest help to me.
https://forum.ih8mud.com/40-55-series-tech/188968-split-case-rebuild.html
You can download the entire manual dealing with all this.
Downloads Index - Page: 1
(it is in there somewhere, not sure what page, but the file name is "H41,H42,H50,H55F_Pub36237E.pdf", you might as well download the 3B manual from there too if you don't already have it)
So my vote would be, unless you have a set up ready to deal with the wheight of dropping and lifting both,
pull only the tcase (saying that, even if I had such a set up, I would still only pull the tcase). Once you have it all cleaned up and you have figured out how the big metal jigsaw puzzel goes together- getting it back on under the truck with the tranny in place really isn't that hard- if I can do it, anyone can.
Good luck.
Hans.
btw, I also managed to do all this with no pullers, no press, just carefull prying, levering, tapping and hammering (cushioned with wood or copper), a bit of ingenuity, on my work bench and woodworking vice. A puller would have been nice though- a press less necessary IMO.