Can you imagine the warranty claims on a two speed rear end from people not knowing how/when to shift it. You would be hearing the grinding of the diff in every neighborhood. LOL. I had a friend with me in a single axle dump with a split rear, he said " I could do that it looks easy". I told him, no I just make it look easy. He didn't believe me, so I tried to teach him. After an hour of him trying, he just couldn't get it. Some people can't. I couldn't imagine him trying to figure out an 8LL with the high/low selcector on the front, and thumb switch for deep reduction let alone an 18spd and splitting every gear.
Driving 8LLs 18spds and 10spd (I despise the Mack 10spd, worst shifting tranny ever) is a skill. Doing it effectively and without damage, even more so. So many companies are going to autos as its easier and cheaper to hire a wheel holder than a driver. With the power to weight ratios of 1ton trucks, I don't think there is a need for 8spd trannys. They have the power and the rpm to get their weight rolling without rowing through the gears. Even international 4700's with a dt466 and a 6spd will do well enough for most around town or short highway trips, even though they are tapped out around 2300rpm at 65mph.
As for adding gears, the ability is already there, mechanically. With electronic controls, they activate different clutches in automatics to get different gear ratios at the output shaft. I.E. first gear with the OD clutch would give you a spot between 1 and 2. 2 with OD would give you a spot between 2 and 3. With an automatic, you could split gears with the converter lock up too. Run through 1st, lock up the converter and drop the rpm 2-500. Then shift to 2nd and unlock the converter at the same time. Run through 2nd and lock the converter, etc etc. On my Jeep, I had my converter on a toggle switch so I could lock it in 1st gear during long down hill decents while off road, thus taking advantage of the available engine breaking. When driving Round the streets, I would play with it and turn my 4spd into an 8spd by locking the converter in each gear and unlocking it at the shift I to the next highest.
I must add, I did love the T800 I drove with the 550 horse N14/8LL combo. That Cummins with dual 10" unmuffled stacks would shake windows...something about the "pop" of the N14....
Just like everyone else on here, I have stuff that cuts, whacks, mows and blows. And way to haul it around.