The speedometer is reading lower than the actual speed. What controls this meter reading.

For strangeness like that you need an auto electrician. The scan tool computer can tell which controller is faulty.

https://www.google.com/search?q=auto+electrician+near+me

You have a unique issue and that is unusual.

https://www.google.com/search?q=2005+Jeep+Grand+Cherokee+Limited+5.7.+No+electrical+power+to+rear+of+vehicle

....

Wheel sensors are usually for ABS and traction control systems. Speedo is usually connected to transmission
https://youtu.be/TWUqx0FrLtM

The speedometer on a 2004 Grand Charokee gets its signal via the PCI data bus from the ABS control module. Both rear wheel speed sensors are used for the speed signal.

Without a Chrysler diagnostic/ calibration machine, I don't think you can do anything. Have you tried disconnecting your battery, wait 10 minutes and reconnecting it?

In the old days speedometer use to have cable today's cars n trucks have a speedometer sensor on the out side of the transmission there not hard to install just google on line it tell how n where it is good luck

Somehow the cable and assembly for the speedometer has become dis calibrated for lack of a better word, U might want to check if the meter has a calibration adjustment on it and if so resetting it for zero with a still vehicle.

It could be a bad cluster, but first try the less expensive check of the wiring and VSS (vehicle speed sensor).

here is the short version...

the vehicle speed sensor is actual an interpreted signal from the ABS module and the ABS module uses the wheel speed sensors

therefore, if the ABS module sees a problem with any sensor it will affect the PCM input for vehicle speed

another possibility, but worst case, is the ABS module has failed

in either case you will need a professional scan tool to monitor actual wheel speed versus PCM vehicle speed

check all the relays under the hood, then check the speed switch for the ac fan control.and wires

YEP BOTH CAN MEAN THE SAME.BUT REAL NAME IS VECHICLE SPEED SENSOR.EVERY VECHICLE YOU LOOK UP YOU WILL SEE VSS SENSOR.