If you're not already there, check out
http://www.thenewx.org. Lots of great technical information there.
Visit this link to see Automotive Customizers offerings on 2005-2007+ Xterra Suspension packages.
Follow this link to see Calmini's 2005-2007+ Xterra offerings. Personally I would not buy from Calmini, their products are inferior, and they've got the worst customer service of any company I've ever had to deal with in my entire life.
Spencer Low Racing (SLR) offers a 3" and a 5" and a 5" coilover conversion for the 2005+ Xterras. They aren't listed on the website yet, but
Follow this link for some information on the 3".
Daystar also makes a 2" spacer lift, but it won't give you any extra capacity.
The AC lifts are the least expensive, but also the least developed. The SLR lifts are the best you can buy, but they're also extremely expensive. The only suspension differences between the Standard and Off-Road model Xterras are the Off-Road has a slightly higher spring rate for the springs (additional leaf in the rear) and the shocks are stiffer. Any 05+ Xterra lift will work on either model. Just specify that you have the Off-Road when ordering.
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