Your gearing wont change. My truck came factory with a limited slip in my 7.5 axle. I actually just sold the limited slip with the gears from the axle to someone because i upgraded to an 8.8 since i have 33x12.5 BFG Mud Terrain KM2s.

You could buy a used one from a junkyard but it will take forever to find one with the correct axle code. If you're buying from a salvage yard you'll still have to replace the carrier bearings and I'd highly recommend rebuilding the entire carrier. I'd just buy a new one (listed below) and get the bearings pressed on somewhere to save yourself the headache. There aren't "trucks that usually have a limited slip 7.5" it was mostly random but i have a 99 with the off road package and a 3.0L motor. Look on the tag on the differential cover and look at the tags. if it says 4L10 the L is for the limited slip. make sure it says 75 on the tag too.

Here is what i have in my new axle but for the 8.8, not 7.5. This is what i had factory from Ford on my truck
Ford Racing M-4204-C75 - Ford Racing Traction-Lok Differentials - Overview - SummitRacing.com

Along with that, you'll need to go to Ford and order carrier bearings and races for it (regardless if you're going used or new). Id recommend also replacing the outer axle seals, ford has them also they're not expensive.