I'm not sure I'd mess with the drive shaft. I'd be a little surprised if you got a solid 2 mpg gain anyway.
Probably driving 70 or 72, rather than 78-9 mph, would give you a nice payback in the MPG department, on this or any other vehicle. I realize that with 4000 miles to go, you generally want to go faster, not slower.
Colo Springs, CO