another big factor is cost. i've heard anywhere from 800 -2000 bucks to change the ring/pinion.
That's more than most of us paid for the skoolie to begin with.
If you increase you top end speed, you most likely won't improve on fuel economy at all, but lets pretend we're in a perfect world for a few moments:
assuming you get 8.5 mpg now, and it was increased to an unlikely 12 mpg, you would save about 34 gallons every 1,000 miles. @ $1.50 per gallon of diesel, that's $50.00 in fuel savings/1,000 miles traveled.
if it cost $2,000 for the rear end swap, you'd have to travel 40,000 miles to make up the difference you'd save in fuel.
If you planned to keep your skoolie for a long period of time, perhaps the $2,000 would be worth it. The top end speed I think is more beneficial than the improved fuel economy.
Quis custodiet ipsos custodes (who will watch the watchmen?)