The Super 35 from Superior Axle is a very worthy upgrade, it definitely allows a Dana 35c to be locked with up to 35" size tires without much worry. I swapped my Dana 35c for a Dana 44 but would have no qualms running a Super 35 in really tough terrain.
In fact, the 30 spline hardened alloy shafts that come in a Super 35 kit are slightly stronger than the OE 30 spline shafts used in a Dana 44. Plus the axle tubes used in a TJ D30 and TJ D44 are identical. So other than the R&P gears being smaller in a Dana 35 which is really not a huge deal for 99% of wheelers, there's not much to not like about the Super 35 kit.
If swapping axles is no big deal and you can find a TJ Dana 44, that would be a good way to go but as said already, the Super 35 kit is entirely worthy and it survives in some very rough terrain. I know of several Jeeping friends that wheeled in positively brutal Johnson Valley with Super 35 equipped TJs. They often broke other stuff but not their Super 35 axle shafts.