It'll probably depend on which model 1991-1997 you end up getting. I've heard that on stricly highway trips it's gotten as good as 16-17 mpg (on a 93-97 I think). Probably be close but a less on a 91-92 model.

You might want to check on www.ih8mud.com. A few of those guys either have done it or gone the manual hubs in front w/driveshaft removed.