For those of us who are ignorant about driveshafts but like to learn things, can you explain what you're wanting to accomplish by modifying this?
Stronger parts? Easier to field repair? More capability? Something else?
In short, yes to all.
So a slip yoke on the transmission like we have gets weird at high angles (lifted vehicles). They apply force at odd angles under hard load where the slip splines may not actually slip like they are supposed to as the rear suspension articulates. You are also very limited on the amount of slip available.
Most hard-core offroad cases (NP205, D300, ATLAS, ect) use a fixed rear yoke and move the slip to the driveshaft. This keeps the forces aligned with the splines so that most of the pressure is applied along the log edge of the splines. This helps prevent binding under high load such as off-roading. You can also get shafts with extra long slip sections to allow for much more articulation without fear of pulling the slip yoke apart.
They also generally use a double-cardan joint (incorrectly called a CV most of the time) at the t-case output to allow higher angle operation than a single u-joint will provide. In that situation, you just pitch the pinion up to just below parallel to the output (to allow pinion rise during acceleration/loading) and the vibrations are usually completely removed from the system.
Lastly, these parts are all readily available on parts store shelves across the country. If you waste a u-joint, you just walk in and grab one from whatever parts shop you can find. Also, the double-cardan assemblies are pretty stock standard parts so you can usually source them from a junk yard in a worse case scenario. For instance, we use a 1330 u-joint at the t-case end of our rear drive shaft. If it was a 1330 based double-cardan, the rear drive shaft from most Ford trucks from 1975-1980 use the same assembly so you could just swing by a junk yard and grab those pieces to try to get yours going again. 1350 is even better since Ford uses them in the front shaft of the Super Duty trucks up to like 2010-2011. The only limiting factor would be a flange or yoke failure on the t-case or differential.
However, we don't have that right now. We have a Nissan specific slip yoke, Xterra specific CV at the rear diff and Nissan specific pinion splines on our rear axle gear sets which make it exceedingly hard to source pieces to convert to the much more desirable rear driveline setups....