99 dodge

are you sure it is "down-shifting" if the Od light comes on, that means it is going into Overdrive, you will notice that your RPM's drop, this gear will save you some money on the highway. at what speeds does this happen? but if your rpm's are dropping when this happens, it is just your truck doing what is meant to do.

This truck has a button for overdrive. You've got a problem either with the TCM/electronic overdrive control inside the trans, or a problem with the overdrive switch/circuit having a mind of its own. It shouldn't go into overdrive like this. Have this checked out.

The wires like to break inside the shifter. Or if you don't use the overdrive you can always just unplug it from under the dash