Changing the transfer case is one thing. Enabling a locking transfer case in a 2003 is a different task. You need to change the shift lever and plate and you also need access to the wiring harness if you want the dash lights to work correctly. Those things can only be done from the top.
I also think it's much easier to take out the centre console than to try to work around the t-case to get to the upper bolts from the underside. There's also the issue of correctly aligning the t-case as you reinstall it without it hanging on the transmission output (this is not so much of a concern removing the old case from a junked vehicle, of course).

 

The shifter can be unbolted without removing the consol. I personally didn't care about the dash light so I didn't wire it. The easiest way to remove the transfer case is to remove the bearing plate on the back of the case, pull out the gear set and remove the output shaft. Not only does it give you much more room to work with but it also protects the input seal from getting damaged.

 

Yes, it's possible to remove the shift lever from above with the console in place. The cover plate in the 2003 doesn't allow for sideways movement of the lever though, and in order to change that you either have to do some blind surgery from above, or take out the couple of bolts and switches and remove the console so that you can get to everything much more easily.
I don't know if pulling the gear set will affect the pre-loads and other alignments inside the transfer case. Regardless, removal is one job. Installation (and conversion of the truck from non-CDL to CDL) is a different process.

 

The 04 Shifter bolts into the 03 without modification. The rear cover is doweled and has no shims so will not change any pre load or alignment.

 

The shifter bolts on, but there was absolutely no lateral movement of the lever possible on mine without widening the cover plate aperture.

 

The 04 shifter is hinged right in the middle so no need for lateral movement. Works just fine.