Likely cost prohibitive. That would be a major undertaking. I have a '99 K2500 (same body) and while I wish sometimes I had that rear door, or even two, in the long run I feel a bit safer after watching a few crash tests on the reverse opening rear door multi-door extended cab pick-ups like what these trucks would have had. This kind of cab is much stronger than the back door models. It's a bit scary to see the 3 and 4 doors fold in a heavy crash. I think that's why the manufacturers have pretty much done away with then and gone to narrow traditional opening rear doors. That b-pillar gives a whole lot more rigidity. The photo is a ford but I've seen similar for GM trucks of the era.