These are the absolute best shocks you can put on your 05+ Xterra or Frontier for around $125 each....period! We sell a ton of these because they work and ride well. These shocks are sold as just the shock (no spring or upper mounts, you can reuse your old parts and 99.9% of the time the stock parts are in great shape) A good spring compressor makes the job a snap. if using the acme-thread type, please use 3 instead of 2 legs, as it will make the job way easier. Sometimes the stock shock stud bolt will be seized to the shaft and cutting the nut off the end of the stock shock with a saw is easiest...this is rare but important for you or your installer to know. 
These shocks offer 5 height settings...0, .5, 1, 1.5, and 2 inches of lift. On the 4wd Xterra/Frontier we dont suggest more than the forth notch or about 1.5", you can set it taller but the ride starts to degrade in our opinion. On the 2wd you can run them at any of the settings. Most Frontiers sit 1" lower in front and these are an easy way to level it and improve the ride. Xterras typically sit level.
These shocks are slightly longer than stock. Bilstein made them as long as possible without any coilbucket contact, we dont suggest adding spacers, ANY SPACERS, to these unless you have aftermarket upper a-arms and then it gets pretty specific what thickness spacers can be run as well as cam bolts, and taller bumpstops. We do sell a kit that uses our 1.5" lift spacer and SPC upper a-arms that will yield 3" of lift and offer a great ride with no noise or alignment issues. All that being said, using these shocks as they were intended requires nothing else be added.
We do not suggest running these w aftermarket HD springs, the ride can become harsh. 
There has been some install issues concerning the proper placement of the washers included with these shocks, the new instructions cover this. Simply, you will reuse the factory upper washer and the new bilstein washer goes "inside" the spring. it may not make a lot of sense here, but you will realize during installation. IF the new washer is used to replace the old washer, the shocks will install just fine but after a week or so the shock shaft will push through the bushing assembly resulting in no shock damping and a bunch of noise. The shock is still 100% fine but the spring will have to come off and a new washer installed...not the end of the world but a pain in the ass none the less.
We will also answer any questions you may have on these if you bought them trough PRG. If you decided that they were a dollar cheaper somewhere else, and are having issues, please call somewhere else for assistance. We generally have lots of these in stock, but they usually drop ship from a bilstein warehouse. We will call right away to let you know if they are not available. International shipping is not a problem, but you will have to pay extra and wait longer for that.