The vents aren't so bad back there.
You just have to realize the rear responds similar to front - meaning if you have it venting on both footwell and up-top vents, it'll reduce the pressure a little.
What most people don't seem to be aware of is that the 5th gen 4runner still has rear vents UNDER the front seats, down by the rear-passengers feet. So air is coming out there, too. Even with the a/c on. So to get more fan pressure on the rear armrest vents, you have to switch the a/c to blow just out of the up-top vents.
Overall, the pressure and amount of air it flows is fine.