You'd have to go with the u-joint outer, and an XJ inner axle. The CV axles wouldnt work as the inner shafts would be the wrong length. I'm sure they could put something custom together, if you want to spend a grand on shafts... but why?
How often are you having quality unit bearings go out? I am told that I'm exceptionally hard on my rig, but a good set of Timken hubs still lasted me a few years (I haven't run unit bearings in 5 or so years now, though).
An upgrade perhaps, but its spending a lot of energy to not fix the weak link.. put that effort into a good truss instead, and gusset the inner C's while you are at it.