In the type of vehicle you are referring to:
A Multi-speed rear end (Gear Vendor's fine products) would do almost the same thing as the 5 and 6 speed Allison transmission does by locking out 1 or 2 of the overdrives. The result is more engine RPM's when overdrives are locked out than when they are engaged to have the same vehicle velocity. Not an exact relationship but very close!
What does a 2 speed axle or rear end do? When in low range it requires higher engine RPM's to turn the drive wheels at the same RPM which means the same MPH.
The general rule of thumb is to keep the engine RPM as low as practical when driving at highway speeds. Easy when driving unloaded. More weight being moved, climbing up hill, bucking a head wind or an increased wind resistance on level ground requires more force to the drive wheels than when unloaded. An internal combustion engine requires enough fuel in the cylinder to explode. Too little fuel (lean) and there will be a low power explosion or no explosion at all. The faster the engine crank rotates, the more fuel it consumes. After engine gear reductions be it a 2 speed axle, transmission gearing, or rear ratio controls engine RPM at a given vehicle velocity. It's the BTU's (heat) of expansion in the correct fuel/air mixture ignited factored by compression ratios that controls the amount of force driving the piston down/away which turns the crank and the drive wheels. Less fuel, less force and less RPM.
