If you're set on running the 7.5, 4.56 gears are available and wouldnt be too bad, but 4.88's would be the optimum set-up. Unfortunately they are not available for the 7.5 rear, nor is the 7.5 an axle really worthy of upgrading. I would source an 8.8 out of another Ranger (bolt in) or out of an Explorer (stronger but some minor welding/work required) and upgrade one of those to 4.88's.