a major contributing factor of a thread like this' strength comes from length of engagement too, so far looks to me like maybe less than 1/2" thread engagement. I'm thinking maybe tig welding this seam is possible.
this end caps threads are probably kept as short as possible to allow the shock to adjust to as low as possible, but that lower cap should be designed to have maybe double that much thread engagement for a big heavy vehicle like this... not only the vehicle being heavier but in a case like this also the unsprung weight and wheels can be much much heavier than a typical import tuner car, on the extension where this failure seems to have happened this shock would have had to counteract the force of a pretty heavy 24" wheel slapping against it at full extension, it's like trying to pull the end cap out with a larger, heavier slap hammer... this isn't a car that's going to have a 17"-18" wheel with a low profile tire, much higher forces pulling against it with really big wheels