Why not a 2 speed or quick change rear end option

Wouldn't Hi, and Lo 4x4 be the same thing? Of course. You would need manual locking hubs

Gear Vendor makes a product that does exactly what you desire. I put one in a previous class C with a three speed automatic transmission behind a Ford 6.9 diesel. It was a great addition.

Alison makes 2 speed rear ends. You can not shift them when driving like you want and would give the the best of both worlds.
A couple of companies make a 2 speed gear box that bolts tot he back of transmissions. I don't know how they would work with autos, but I had one on my 68 C10 3 speed on the column and loved it.

Last I read the major manufacturers were only going to offer autos in there HD lines. I don't see them making anything over a 6 speed auto because of size. I could see a 8 or 10 speed if the made split shifts.
Size wise on a manual would also be huge at 9 or 10 speeds unless it was a split shift.

Many years ago, such rear axles were available. They were quite common in Fords after WWII. They were Columbia 2-speed rear axles. They were GREAT!
Two and three speed "Brownies" (auxiliary transmissions) and rear axles were very common in trucks from ton and a half or two ton on up. Years ago, I drove a Ford F-4 (IIRC) that had an electric shift two speed rear axle with that horrible 4 speed straight tooth transmission. I was young then, and double clutching didn't bother me.

The advantage of a 2 speed or quick change rear end is that when you are towing you could pick a rear ratio that gives you 6 useable towing gears instead of 4-5. It would also give you more available power at most road speeds.

When not towing you would be back to a ratio that gives you good fuel economy and lowers rpm for empty driving.

The gear vendors stuff is nice and also expensive.

But with the coming 9 and 10 speed transmissions soon we will have most of these advantages.

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.

Maybe not exactly what you are referring to, but Gear Vendors has something that might work.