2004 transmission and speedometer problem

For the 1999-2004 model year the OSS sensor output goes directly to the PCM. The PCM then "tells" the cluster what speed to display.

So how to trouble shoot this it's important to know what the PCM "thinks" the car's speed is. Is it high/low/correct?

Consider that IF the PCM doesn't know the car's correct speed, THEN the automatic transmission won't shift correctly.

Do you have an ODB2 scanner that is capable of monitoring operational data? If you did it might make short work of your problem.

OBTW, an OSS sensor is NOT a 5 volt VREF sensor. It generates it's own AC output. So if there's 5 volts at the OSS sensor there's already a problem.