Although I'm new to TJs/Jeeps, and even solid-axle vehicle technology, which limits my vehicle-specific failure knowledge, I have been involved in failure analysis of vehicle components for a few decades. That's my caveat for possibly missing something obvious that a more experienced TJ expert will see immediately. Based on that, there are several identifying marks in the broken surface that give me clues as to what's happened:
View attachment 328129
The discolored area (red arrow) looks like corrosion, indicating a long-term fatigue (it cracks a little at a time until the "final straw breaks the back of the camel"), rather than an instantaneous, failure (on big "boom"). That means the break started at the top (blue arrow) because it's going to start on the surface, and when it does, water starts the corrosion of the base steel. The beach marks (so named because they resemble wavy sand on the beach from waves/tides) indicate that the failure was due to bending, in the direction as though you held the shaft firmly and pushed down on the end of the mounting bolt shown.
That said, I'd check three things:
- Does the damper still function smoothly (no internal failures)? An internal failure could cause the damper to bottom prematurely, which could induce a slight (or even not-so-slight) bending moment on that shaft. An internal failure could also increase the damping force exponentially, but you should have felt that as increased effort on the steering wheel.
- Are the mounts restricting rotation of the ends, putting a bending moment on the shaft? My bet is on this, since the broken mount seems not to have much ability to rotate as the suspension articulates. Since the amount of times your Jeep went to full bump or full rebound is much lower than the amount of times spent somewhere in between, it lasted for years with this issue. If that mount doesn't allow the damper to freely (or mostly freely) pivot, it's putting a bending load on the shaft, but not enough to break it the first time it happens.
- Is it possible that the damper just barely bottoms out before anything else? If that happens, it could also put a bending load on the shaft, but because it just barely happens, the loads are small enough that it doesn't break the first time it bottoms.
My opinion is that is not a typical failure, and it'll repeat if you replace it exactly like that. I'm interested to hear from the gurus...