One problem related to axle assembly has been reported for the 2004 Mercury Mountaineer. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2004 Mercury Mountaineer based on all problems reported for the 2004 Mountaineer.
Rear axle failure at 11,000 (side bearings) and 48,000 miles(bearings and rear axle). Due to design mistakes, and probably cost reduction programs, mud can trap open a breather valve for the rear axle. Then water goes through the valve and into the axle, and destroys the side bearings,and clutch flange,and pinion. The replacement cost of these items is nearly$1,300.