The longer the vehicle is driven while slipping the more damage will be done that may require tearing the transmission down to replace bands and clutches.
The slipping can be caused by two problems. The first is sticking shift valves in the valvebody. These valves are actived by electronic controled solenoids. If the valves are sticking, they casue delays in shifting out of gear and shifting into the next geat and that causes the slipping. Also, when the valves stick, they create an excessive load on the solenoids making the weaker and weaker.
Just replacing the valvebody will fix the problem temporarily, but the solenoids are weak and will fail soon. It's best to replace the valvebody and the solenoids.
The other problem with the Sport Trac/Explorer 5 speed auto trans is a weak valvebody gasket. If the gasket starts to leak, you will have weak fluid pressure making shifting slippy and sloppy.
Ford put aout an improved valvebody to remedy the sticking and the failing gaskets. I don't know about the part numbers or how to tell you are getting the improved model, but at least you will know to ask.
...Rich
