I can't answer the question as to weather or not keeping your stock gears will over time mess with your transmission. But I can tell you from experience on my personal trucks and as an installer that, you will love a lower gear ratio. I think 35's are the breaking point for needing lower gears. 4:10's would make you fall in love with your truck again. + it's bolt on horsepower from where your at now.
09 F150 2WD DD Pre-runner build on a budget. 93 F150 SAS - 9" springs 3" body - 9" conversion - 4.88's - 39.5's