Very hard, and very expensive. Shops charge a lot on labor, because it takes a good amount of skill to do it. We are talking over a grand here.
If you are really pressed on cash...there is another option. While many will disagree, you could always grab a d30 and d35 from a 4 banger. They are identical to your current axles, but are geared at 4.10. This will be about good enough. It's not perfect...but better than driving on 3.07.
I hate to give that as a solution, because it's more of a temporary work around. It really depends on your situation, ability, and the kind of wheeling you're going to be doing. Also depends on how cheap you can find axles, and the condition.
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