I've always wanted to have a 2 speed rear end to try and get better mileage. Seems like if you could lug the truck down to about 18-1900 RPM at 75 MPH, you would get some good mileage. Wouldn't be able to pull much, but then you could switch back to the other range. I looked into it on another truck I had. It was 4x4, but wasn't a big deal to just use 4x4 in just one range. Would need some sort of lockout to keep me out of 4x4 in the other range
I found one that seemed small enough to work, but a guy told me that you couldn't put a locker in that one. Don't know if it was true or not. Not much help to you, but at least you get a bump to the top. Let us know if you find something.
Chris