Is the Dana 35 a weak spot? Yes, under certain conditions. Is it a ticking time bomb? No, this is another exaggerated internet story, told by those with no experience. Ask yourself this question. If the Dana 35 is so bad, why was it put under tens of thousands of TJs, across a decade? In the grand scheme, it has proven to be very reliable.
That said, seals leak and parts wear out over time. Replace any worn out parts and inspect for other issues. Upgrade only if your intended use necessitates it. Otherwise you'll spend a lot of money for little to no perceivable gain.