I doubt you'll ever recover the cost involved to do the gears, especially on a truck w/ over 125K miles already. I'll let you do the math but that ~$2K will buy a lot of gas. Figure 2 mpg increase, tops. Beware, this is an experiment, you mileage may even go down. 22 mpg is excellent for a 4x4, you may have already found the sweet spot. Lowering the RPM's by X% doesn't automatically mean the MPG will raise X%.

Have you recalibrated the speedo for the bigger tires? Your mileage can be off if not. What do you mean by "raised" TB?