Well, that escalated quickly.
A couple o' observations: The rear axles are not identical between 2WD and 4WD. The shock mounts (at least on my truck) are different heights. That's not to say whether it's more cost-effective to change the tube length vs just welding on different shock mounts, but GM is inventorying two different parts, so it's not for cost or convenience on the line.
In racing, one of the things we learned about track width was that a narrower front track was a higher rollover risk than and equal F - R track width. Think of a tricycle. Is it possible that GM compensated for the higher COG of the 4WD truck with a wider front track to reduce its rollover potential?
Lastly, remember that in engineering there are no design flaws, only features. So if it was actually done for cost reasons or was just a mistake, the consumer is told it was for improved somethingorother.