You are literally dropping the 2:1... taking it out. So, (ignoring the unused 1st gear) it turns a closer-ratio 6-speed transmission into a wider-ratio 4-speed (across the same 0-65MPH driving speed range).
The fluid torque multiplication of the torque converter was there before, and is still there. You're not "transferring" anything to it, or activating it where it was inactive before. The truck just had lots of torque (e.g. for going over a 24" curb, which is the spec requirement), that wasn't frequently used, and now it has 1/2 as much (literally 1/2, because the 2:1 was removed).
None of this is likely to change fuel economy much. People always claim things improve MPG (e.g. aerodynamic kits, raising tire pressure, etc.), and there is a kernel of truth to all of it. What makes it a "kernel of truth" is that while it's possible to make tiny (0-5%) improvements, you can't beat the physics and get much more. This has been well studied across the entire automotive industry, including by the Army for these exact specific trucks (multiple times, one of which found that a 12% gain could be had by going to a 12-speed, not down in gears).
Immediately, if someone (novice!) is claiming more than 5% gains, red flags should be going up.
The military studies (as well as all similar commercial truck studies) show very clearly that the most inefficient part of the whole system is the engine. It wastes 70% of the fuel, turning it into heat, meaning that ALL OTHER FACTORS COMBINED can do no more than affect the remaining 30% total. In other words, if you could make all other losses (e.g. wind resistance, transmission friction, tire rolling resistance, etc.) go away completely, the best you could do is improve fuel milage 30%. Ever. Period. Full Stop.
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As a side effect of the 2:1 hub reduction removal, the drive shaft speed is cut in 1/2, which is good because these shafts operate close to max RPM. However, that means the drive shafts, u-joints, and axle shafts are taking 2x as much torque, which is definitely bad offroad.
Naming the hub change "Eco Hubs" is cute, but snake oil. The bottom line is that if somebody claims more than a 5% MPG change you should be suspicious. If they start talking 10-25%, you can be fairly certain they are wrong, without even looking at it. And the reason is physics... well understood, thoroughly studied, endlessly documented. But there is always some quack that is sure he's got the next big thing, if you just know the secret sauce he knows.