Have a 1994 plymouth voyager 3.3 v-6. Transmission quit changing from 2nd to 3rd gear exactly at the time my speedometer quit working. It now will not go past 2nd gear at all. Also check engine light comes on occasionally, which causes engine to have more power but has never gone past 2nd gear.
The computer in your vehicle, (ECU) makes decisions to adjust parameters based on sensor input.
One of the sensors is a speed sensor or a VSS. Some will work if speedometer fails, but most will not due to safety related issues.
The problem could be the sending unit, generally on the transmission, the connection between the VSS and speddometer or the speedometer itself.
Let me know if I can help in any way.