CarComplaints.com Notes: The 2005-2007 Xterra appears to have a widespread defect where coolant leaks into the transmission, causing transmission failure around 90,000 miles. Repair costs are typically upwards of $3,500.

The same defect exists in the Pathfinder & Frontier as well. And according to the New York Times & a class action lawsuit, the defect actually extends through 2010 model year. More info here.