In addition to the bad ride, having your upper bumpstops touching is also potentially dangerous, as your tires will leave the road on bumps instead of following them. This could be very bad if you, say, hit a dip in the highway during a curve. You are also running your CV joints at way too much angle all the time. At the very least, your boots will tear sooner (I know this) and allow contamination in the joints even if you aren't actually rotating the driveshafts. As noted, the Xterra is supposed to be higher in the back. Remember too that the rear will settle probably at least 1/2" within the first 6 months.