[maxwedge]

Both diffs have to be done, if you have never done one before, and don't have the proper tools I suggest a pro do this, your truck may not be the proper environment as a learning tool. 3.42 is probably the next size lower, which would allow the engine to turn about 10% slower for a given speed. I think the fuel mileage savings would take a long time to make up for the cost of this project.