What you're experiencing is Z-Fighting, and while the prevalence of this issue could be partially related to drivers, it isn't entirely. There is no permanent solution to this problem, as part of it lies within the game itself.
Although FFXI's engine is capable of displaying the game at higher resolutions than what its configuration GUI allows, the game wasn't tested nor marketed with those settings in mind. Therefore, the models and textures were never tested with them. Because of that, polygons which wouldn't exhibit sorting problems at lower resolutions may at higher ones, as they were never meant to be displayed that way.
To provide a bit of credence, before I became a journalist in the gaming industry, I worked in different aspects of it, including various positions involving development. A lot of problems were revealed in testing using resolutions that were ultimately listed as unsupported, while some were just left alone, as they were too complicated to fix. This included big games like "Call of Duty 4," to note, so it's unsurprising FFXI would have the same issue, especially considering its sheer size.
I run at high resolutions, and occasionally get flicker on random pieces of gear, or scenery. I personally suggest living with it, but you can indeed try different driver editions, and see if it changes somewhat.