I did some research and discovered that my rear axle was a standard spicer part, and that my current gear set was a 4.78. I found that ratios went all the way down to 3.54 for this axle unit, and that rebuilt differentials were available.
I was concerned about going to such a numerically low gear ratio, but I did some math in excel to ascertain effective changes in torque to the ground at a given RPM in each gear, and felt that the swap would be doable.
The goal was to be able to cruise at 60mph right in the engine's powerband - around 2000 rpm. This would theoretically give us good highway characteristics and fuel economy.
After having the diff swap done, the rpms were reduced dramatically. Driving around town is easier, as you don't have to grab the next gear so often. Highway driving is much better; on our very long 2012 trip, which included many mountain segments and high speeds (max observed speed: 90mph downhil in Wyoming), we averaged 9.8mpg for the whole trip. This is a tremendous improvement.
The downside is that mountain climing in the bus is limited to 2nd gear in many cases. This means Vmax of 25mph. When shifting from 2nd to 3rd on a steep uphill segment, the rpms drop too much and the bus cannot pull hard enough in 3rd gear. This can frustrate other drivers, so it's important to check your mirrors and always pull off to let people by you. I may attempt to upgrade the power or change turbines so that low-end power increases, but I'm so happy with the mileage I get currently that I am hesitant to change anything.
The total cost to have the differential swapped was $2400. I think the improvement in highway characteristics is well worth it. Note that I have a manual transmission, you may have different results with an automatic.
Eastwood Tools makes an excellent tubing flare tool. The most wonderful aspect of the tool is that it is completely idiot proof. The tool is expensive, but this is one of those situations where if you just buy the tool ahead of time, first, you come out ahead because you save time, frustration, materials, and don't waste money buying tools that don't work.
Here is the link to the tool I am talking about. If you plan on replacing any of your brake lines, I highly recommend getting this thing. Eastwood Professional Brake Tubing Flaring Tool.