The Rubicon's OE shafts are carbon steel 30 spline shafts. There is a major boost in strength available by upgrading to Superior Axle's Evolution Series 30 spline axle shafts which are made from a hardened alloy with rolled (not cut) splines.
Don't bother upgrading the front axle u-joints, they are plenty strong as is unless you are doing some just plain crazy stuff. The reason the OE u-joints fail if pushed too hard is not due to weakness in the u-joints themselves. The root cause of front u-joint failure is actually usually because of the ears that hold the u-joints failing on the unhardened carbon steel axle shafts the factory installs.
The "ears" on the inner/outer axle shafts that hold the u-joints are unhardened. As such, when you put much stress on the front tires (especially when the steering is turned very far to one side), those ears stretch or fail which then spits out the u-joint. That allows the now free u-joint to smash into one or more of the four ears that used to hold it, breaking the u-joint. Replacing the front 30 spline axle shafts with hardened alloy 30 spline axle shafts like Superior Axle's Evolution Series will prevent that from happening by their ability to hold onto the u-joints under stress much more reliably.
After breaking my passenger-side axle shaft ears and u-joint while doing Sledgehammer years ago, I replaced my front axle shafts with Warn's hardened alloy axle shafts (no longer sold) and even doing tougher trails after that, I had no more front axle shaft or u-joint failures. That was with Spicer's OE size 5-760x u-joints up front.
So don't bother upgrading your front u-joints, upgrade your axleshafts. Then if you get into some crazy-tough stuff later on (REALLY stupid-crazy hard) and start breaking u-joints by themselves, you can go to CTM's uber-strong u-joint. But at $160 each, they aren't for the faint of heart.