Mvbeggs,
I spend half the year or more traveling, mostly the western states. I'm familiar with virtually all the rocky mountain grades. My trailer weighs 2350 on the axle with 225 on the tongue for a combined truck load of around 600 pounds with gear. I have the 4.56 gears with 235/85-16 E rated tires (approx 32"). My unloaded truck weight is 5350 with a full tank, me and 2 dogs. I also towed this combination with stock gears prior to the switch. At 65 I'm turning 2550 RPM in high gear (manual 6 spd). With the 255s you would need the 4.88 gears to approximately match what I have.
With regards to daily driving: I love the lower gears around town. The only downside was that they produced even more axle wrap than I had with the 3.73. That was eliminated with the installation of All-Pro expedition springs.
With regards to highway driving: The 4.0 does not produce much power below 2400, but the good news is that the engine purrs right along at the higher speeds. My highway speeds are never above 70 and usually around 65. That keeps me under 3000 RPM.
With regards to towing: While the 3.73 gears did work, I found the 4.56 much more enjoyable to drive. Launching and accellerating through the gears is much easier, especially loaded on a hill. Down the road the difference is about one gear. With the 3.73 I would be one gear lower for the same incline than I would be with 4.56. Moderate grades require no downshift. I just lean into the gas a bit and it rolls on. Steeper grades like the I70 climb, the Siskyou pass etc, I take in 5th gear running I think around 2850 RPM. The only climbs that get me into 4th and down to 55 are like the climb into the Tetons out of Jackson Hole. On alot of these passes altitude also takes it's toll, but I can't recall ever hitting 3rd unless for traffic or corners.
There is one issue though that is probably not common knowledge. The weak link in the towing ability of the 4.0s is the rear diff. Both the 8" and the so called
"8.4" measure 8". That is a really marginal gear size for the power of the 4.0 under high loads. When I swapped gears after just 20K of combined towing my carrier bearings were already showing evidence of spinning. To monitor the situation I installed a temperature probe in the rear diff during the gear change. My typical unloaded diff temp running mostly flat or rolling hills is between 125 and 165 degrees. The same run with the trailer and weight runs 150 to 185. When I start climbing hard the heat builds fast and often runs 200 to 220 degrees. I don't let the temp ever exceed 225 degrees. If I approach that I get a lower gear and slow down. Above 225 the rate of viscosity breakdown accelerates and at 250 it is cooking the oil. The point is, the truck has the power to cook the diff under these conditions and I would never know it without the gauge. No doubt I had done that before the gear swap which resulted in the spun bearings. I would advise anyone towing under similar conditions to take it easy up steep sustained grades, especialy in the summer. I change my rear diff oil every year and I'm currently running Amsoil Severe Gear 75/140. At the last change out the oil looked just fine after running all over Wyoming, Montana, Idaho, Washington, Oregon, and Northern California. The trick is not overheating it. I could probably go longer on a fill, but I know the gears are working at their limit at times so I spend the bucks and change it out.
Hope that helps....