One problem related to automatic transmission torque converter has been reported for the 1999 Ford Explorer. The most recently reported issues are listed below.
Same 2-3 shift problem. Especially when cold. . . Gas it on the on-ramp, all of a sudden it "pops" out of gear and you're hoping someone doesn't slam into the side of your vehicle. After my last near death experience, I took it to the dealer, they said it was a valve body gasket that was missing? (where did it go?) they said they installed the upgrade kit and put in a new gasket, said it worked better. About 4 months later (when it got cold), it started again. Work & parts are supposed to be under warranty, but the dealer "thinks" it might be something else. My complaint is, 4 months ago I paid $900 for them to fix what is a common problem with their product, the exact same problem is happening again and they are looking for ways to get more money out of me, or worm their way out of fixing what I've already paid them to fix. The technician suggested it might be an internal leak, the torque converter, then suggested I might want to replace the tranny. My response was you had it torn apart last time, you didn't fix the problem, why should I trust you now? I don't subscribe to the theology of "repair through attrition". This is a serious problem that Ford should address or be sued for.