At 60 mph you will be going 63.4 with 37" diameter tires.
http://www.chrismans.com/Calculators.html
And this from
http://www.4x4connection.com/gearchart.htm
:
According to this chart, the acceptable RPM range is 2000-3000 which is represented by the entire shaded area of the chart. The optimum RPM range is 2300-2600, and is represented by the light yellow shading. When changing ratios to improve mileage, choose a ratio within the dark yellow shaded band below the optimum range.
Black Lingenfelter Lux