First thing is do a little searching, this topic has been covered many, many times.
Here is a summary:
the good- shifts great, easy to split gears, supposedly low frictional losses (good for double overdrive), install is easier and cheaper
the bad- run hot, higher maintenance, real problem with reverse loading (engine braking in OD and low speed reverse not in OD, like backing your camper up a hill), costs more, have to lockout 4x4 to engage (should be no problem)
US Gear Dualrange
the good- built like a tank, 4x4 high and low no problem, reversing no problem, engine braking no problem, comes in either high or low versions (if you have 3.55 gears you would want the under drive version, 4.10 or higher you would probably want the over drive version), has a higher rating than GV
the bad- IS NOT BEING MANUFACTURED, shifts are not as easy and quick, has higher frictional losses, so DOD may not make sense for economy, 4x4 install requires moving the xfr case and messing with both driveshafts
If the DualRange was still in production, I would recommend the underdrive and 3.55 gears.(this depends on your loads)
If you have 4.10 gears (and you have a 250 or 350), then the GV (this depends on your loads)
The best options (for economy highway speeds):
More gears in the transmission
a true 2 speed rear
Everytime a gear is engaged there are losses. Among others, there is a loss in the trans and a loss in the rear. The trick would be to get your preferred gear ration without introducing another loss, ie the overdrive.
So Smokey I hope your friend is building a 2 speed rear, cause this would be the best option.