If you were going to consider changing gears I would probably go with a 3.42:1. That would get your RPMs down to about 2,800 RPMs @ 75 MPH with your 31" tires. However, you have a 4WD so that is 2 sets of gear changes! Just the gear sets alone are going to be $600! Then you probably need to pay someone to do the work. That will be at least double I am guessing...so something like $1,200?! - This gear change will be decent on the highway. It won't be super-low RPMs but it will be okay to drive like that for long trips. My '69 Chevelle turns about 2,600 RPM at 75 and it is decent. You would be better with something like a 3.36 gear if you can find it.
If you wanted to simply change tires you would need something like a 35" tall tire to make that difference, but then you are going to have to lift the truck to fit them under and you are looking at a 6" lift and new tires. Tires will be close to the cost of the gear change, but add the cost of a lift!
The other option is changing the transmission or adding a Gear Vendors overdrive. Neither of those options is cheap.
The least expensive option is slowing down and taking more time to drive your commute!
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