1996 Ford Thunderbird PO720 Error Code - Speed Circuit Malfunction Diagnosis

My 1996 Ford Thunderbird with a 4.6L V8 engine is displaying a PO720 error code, indicating a speed circuit malfunction. The speedometer has stopped working, and the overdrive light flashes intermittently. While driving, the vehicle fails to shift gears properly. We have already tested the transmission and confirmed it is functioning correctly. The transmission control module (TCM) and speed sensor module have both been replaced without resolving the issue. What could be causing this problem? Any insights or troubleshooting steps for a PO720 error in a 1996 Ford Thunderbird would be greatly appreciated.