The mileage on these plugs is unknown, as this is a used vehicle w/o a complete service record. All 3 plugs on the driver side (4,5,6) were almost completely eroded. The 3 on the passenger side (1,2,3) however, showed very little sign of wear. Is the driver side fuel/air mixture more lean than the passenger side, thus making it burn more hotly? Is it even possible for one half the engine to get a different fuel/air ratio than the other?
Looking at the two V6 engines availible for the 97 Ranger, both have 2 seperate fuel rails and different o2 sensors for each bank. As stated by @Ukko in the comments, If the o2 sensors aren't agreeing with each other, it could cause one bank to run more/less lean than the other. The same could be said if there is some problem with the fuel rail. Or maybe even a previous owner had some reason to replace one bank of injectors only...
I think the next best step would be getting it hooked up to a scan tool and getting fuel trims (short and long) and o2 sensor readings for each bank. This would go a long way towards narrowing the problem down.
A visual evaluation of spark plugs can be a window into the combustion process. These plugs show that the color is normal and similar bank to bank. Plugs that are dark and sooty indicate a rich mixture. Plugs that are very clean with light colored deposits can indicate a lean mixture. Oily wetness can indicate oil in the cylinder. While color alone does not definitively confirm proper mixture it should be good enough to keep the plugs cool enough for a normal wear rate.
Now lets discuss the wear seen here. On one bank the center electrode is worn to the insulator indicating a long time in use or low quality materials with little platinum on the electrode. The other bank has significantly less wear on the center electrodes. However the ground electrodes show severe wear. Taken toghether the two sets indicate that this is a waste spark type ignition. In this system one coil fires two plugs at the same time. The electron current flow is toward the center electrode on one bank and toward the ground electrode on the other. The wear follows the current so one set pulls material from the center electrode during the spark and the other pulls material off the ground electrode. Hence this typical wear pattern.
Iridium plugs last longer than platinum and are suggested for applications such as this. Plugs with the platinum or iridium on both the center and ground electrode are required in these systems because of the electron flow.