Parrsboro, I have a 7 inch lift and 37's on my 1999 f-250 v10 4x4. Dont do 4.56. Do 5.13 or even 5.38. I am doing 5.38 with mine. The 1999 model was the least powerful v-10, and when you lift the truck and add all that rotating mass, you need to gear lower. If you go to www.ringpinion.com they have a tire size rpm calculator to use. Use the one on the bottom and enter a trans ratio of .71, as that is the overdrive ratio for the 4r100 tranny, which is what you have if its an automatic. The V-10 is happiest cruising at 70 mph at about 22-2500 rpms. You will see that the 5.38 gearset will put you at about 2350 rpms at 70 mph. That is the sweetspot! The 4.56 gears will put you down to about 2000 rpms. The consequences of being below the sweetspot are: excessive shifting, excessive trans wear, poor fuel economy, lack of power, etc... Let me know if you have any other questions.

-Chris