"Voids your warranty" is a pretty relative term--the driveline angles are changed a bit by the shackles so if, say, your rear diff went bad your dealer could try and blame it on the shackles. This was a pretty big hangup for me on deciding on the shackles, but after hearing so many horror stories about dealers shafting people just because their trucks had obviously been offroad, I decided not to care. My truck has enough dents and scratches on it that I'm pretty sure they'll be able to give me hell if they want to, regardless of how many aftermarket products are on it.

Plus one tire sizing probably would improve the handling, however it's the opposite of what most of us have done, namely "plus one" refers to increasing the rim size by one and reducing the tire sidewall aspect ratio so as to have less tire sidewall to roll through corners. If you offroad, though, you upgrade your tires to gain traction & clearance, often from a tire with a taller sidewall, hence more sidewall flex, hence crappier onroad handling (but not by a whole lot.)

HTH

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gothamist -- MAXC member
http://www.beatdriven.com/xterra.html