The 3.92 and the 3.21 have pretty much the same gear ratios available. The 3.92 has a lower first, the 3.21 has a taller final, the rest they share. By "share" I mean that they have gear ratios that are pretty much identical. Do you think there is a significant difference between a gear ratio that is "3.21" vs "3.2144"? That is the differences I'm referring to.
When you have enough gear ratios available such that you only need 400 rpms to upshift to the next one, a difference of "0.0044" is not even going to change your RPMs by more than, what, 10 rpms? I'm too lazy to do the math. That is the most extreme example, but the other ratios are pretty much identical as well, and none of them are going to be more than 30 to 50 rpms.
So yes, if you're going down the road in your 3.92 rear end at 2100 RPMs, and me going down the road at same speed at 2130... nobody cares, it changes nothing with respect to power or fuel efficiency. Yes there is still a tiny difference, but it is completely insignifcant to the point where other factors are much more important (tire pressure, weight in the truck, top tier gas vs crummy gas, octane rating etc etc).