well since you've got a 2wd I would try 4.56 and if its to low then go 4.30. being 2wd makes it super cheap as compared to us 4wd. You will lose your top end speed but unless you run higher speed rated tires its actually a good thing. Most truck tires will tear themselves to pieces above 140. With 4.56 it should launch like crazy, you will probably have a hard time putting the power to the ground, but if you can get it to hook up no truck on the road will beat you in the 1/8 or the 1/4.

In fact this is kind of kids way to do it but ill look for you and be back with the results soon. Forza 4 is a racing simulator game and one of the trucks is a 2wd f150, ill change the gear ratios and see what it does to e.t.s and 0-60 as well as cruising speed vs. rpms