It seems that while definitely not an immediately necessary upgrade, it is probably reasonable to replace the front drive shaft, so I think I'll do so with the lift.
Unlike on the JK before, I wouldn't say it's necessary up front on a JL or JT. Being that the FAD keeps the front shaft from spinning in 2WD, there is very little wear on rzeppa joints during normal use. In the past and on the JK, the front shaft would have been sitting at a steep enough angle to cause the rzeppa joint boot to be in a constant state of pinch and this combined with constant spinning caused it to fail prematurely. Also, on the JK and at a full flex, the front shaft would have torn it's slip shaft boot on the transmission skid. Well, at least for people who had an earlier model JK and with an automatic anyway.
As for the rear drive shaft, this appears absolutely unnecessary unless severe articulation and/or hitting rocks, so will be holding off for now.
Is this a fair conclusion?
Technically but not necessarily. The factory rear drive shaft is quite large in diameter and depending on just how much lift your kit yields and whether or not enough axle correction is made with the relocation bracket that's provided AND how much actual articulation you see with the shocks you use, you may find that the slip shaft boot will make contact with the gas tank skid and that can damage it. The pinion flange may also contact the skid at a full flex as well but that's not a big deal. Some adjustment with an adjustable track bar ON TOP OF a relocation bracket can help address both.
Interesting. I'm not surprised the rear shaft had damage after those trail runs, but I'm a bit surprised the front was in better shape than the rear. I like that many of the front shafts are pretty much just bolt on. The rear shaft seems more complicated (intimidating?) because the need to change the yolk at the axle.
As mentioned above, things are different on the JL and due to the FAD and lack of a skid that would otherwise damage a factory front shaft. The rear may get damaged for the reasons stated above. Changing out a pinion flange to a yoke is something you should be concerned with. It's easy to over-crush a crush sleeve and that will lead to a fried bearing.