I think it is a couple/three miles per gallon. I've seen other posters talk about the mileage difference. Since this is a daily driver truck, I changed it back to a 3.21 like I currently have on my 2019. Just turning 60, I was thinking we may get a RV travel trailer somewhere in the near future. That's why I originally chose the 3.92. But if we do, it'll be a smaller one. The 3.21 should handle it.
In regard to the elocker, we live in the Memphis area. Ice and snow happen each year, but for the most part only 3-7 days out of 365 (2021 was an anomaly, as we had a foot of snow in Feb on top of an ice storm). It's nice to have the ability to get around while others slip and slide. But I just didn't think the elocker was going to be used enough to build it that way. I've heard you have to turn it on before you need it, vs anti-slip which kicks in automatically. I don't know if that's true or not, but I went without both. The 4x4 with good AT tires should handle the limited winter weather in this part of the country.