Merdas, I face the same issue of too high RPM's on the highway. In the suburbs at 50-55 MPH I get beautifull 24+ mileage so I am very happy, especially after reading others posts (under 20)
Problem starts above 2400 RPM on the highway (which is most in my case) and 23-24 MPG is history. I was also looking to change aspect ratio to lower number but I guess I was going in the wrong direction.
I was looking for the whole new axle, well that is $4000, no labor so I am not into this solution anymore.
I didn't know You can replace just the inner parts (ring, pinion?) to get lower ratio. Even going down to factory 3.92 should be enough for me, that's always 200-300 rpm's less, besides I don't go over 75, seldom over 70 so I should be able to squize 24, maybe 25 MPG. Too bad I didn't know about aspect ratio 4 months ago, when I was buying the van and salesman is the last person to tell (wonder if he knows?) I found this forum little too late, but better late than never, it already has helped me to get to 24mpg from initial 18! and I think it's not the end yet
Anyway, please let me know if You succed changing this pinion so I can try the same