Gears are specific to each axle, so gears from a dana 35 will not work in a 8.25.
Low pinion means that the pinion enters the axle and contacts the ring gear below the centerline of the axle tubes, where as high pinion is the opposite and enters the axle above the centerline. All XJ's with the exception of the '00 and '01 year are high pinion axles.
You most likely have 3.55 gears, which means the driveshaft will rotate 3.55 times to turn the wheels one revolution. To tell your gears, spin the wheel, or have a friend drive while you lie on the ground and watch the driveshaft spin.
Now to the bigger thing. There is a ton of work and specialized tools to set up gears, its no where near a bolt in affair. Not to be mean, but if you are asking questions like these, you most likely shouldn't be setting up gears. But to answer your question, you will need not only the gears, but also a master install kit with all the shims, bearings, and seals you need for the axle. Used gears are even harder to set up, as they have already been broken in for a different axle, and will make noise most likely in a different axle. You need to make sure your pattern is good when you set up a differential, and that includes setting the proper pinon depth, shimming the carrier to get it in the perfect place, setting the backlash, pinion bearing preload, carrier bearing preload, etc. It takes lots of time, and patience.
Its easier and cheaper to search around for a junkyard that has factory axles that already have 4.10 gears in them. Search around for cherokees with 4 cylinders and manual transmissions.
