One problem related to differential unit has been reported for the 1999 Ford Windstar. The most recently reported issues are listed below.
Abs light, o/d light in dashboard coming on. When vehicle purchased as used car these items showed up on the drive home after purchase. Veh has been in to Ford dealership garage 4 times. Brakes were working fine at that time. 3 months later brake light come on. Check and find that brake fluid is leaking from multiple locations. One being the brake pressure differential deactivation switch (2b264) located under the master cylinder. Upon removing this plug I found brake fluid in the connection. Looking below I found brake fluid on the wiring harness below the switch and on the metal panel, where it had actually began to rust. Believe this harness is the power steering wiring. Brake pedal in cab now goes to the floor before brakes are applied. There is a hissing sound when this is done. Believe that the master cylinder is leaking fluid into the brake booster as well and has destroyed the gasket in there as well. This happened while my wife was driving. Thankfully all of this happened without causing an accident considering I transport my 3 children and wife in this van.