One problem related to differential unit has been reported for the 1999 Mercury Mountaineer. The most recently reported issues are listed below.
1999 Mercury Mountaineer. Leak in rear differential at 40,000 miles. Replaced pinion seal. At 67,000 miles, a loud, whine emitted from rear axle when driving (regardless of speed) and the mechanic advises us to replace rear differential bearings or entire rear differential. If we don't, we fear the rear axle will fail, causing a crash.