One problem related to differential unit has been reported for the 2005 Mercury Mountaineer. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2005 Mercury Mountaineer based on all problems reported for the 2005 Mountaineer.
Rear differential needs replaced. Mechanic said can be dangerous. I see that these Ford/Mercury's have this problem all the time. Cost too much to fix, yet I have wife and kids in that car.