Trans problem codes and Delay downshift when stopping and speedo show 20 mph

As Monde said, I would start with the Output Speed Sensor. A check of resistance between Pins 13 and 14 of the Trans Control Module will give you a look at the circuit's condition. Disconnect the connector from the Transmission Control Module and find Pin 13 (It should be a Dark Green/Violet wire) and Pin 14 (It should be Dark Green/Brown). With a mult-meter set to read Ohms/Resistance ( Ω ), touch one lead to Pin 13, and the other lead to Pin 14. A reading of 300 Ω - 1100 Ω is a good reading. If you get anything else, go to the Speed Sensor and disconnect it. Measure the resistance between the two pins of the sensor. Again, a reading of 300 Ω - 1100 Ω is a good reading. If the reading is bad at the sensor, replace the sensor. If The reading is good at the Sensor, But not at the TCM Connector, check the wires between the sensor and module for faults.

Alternator failure is a common cause for transmission related symptoms on these.
One of the the tell tale symptoms is the speedometer registering speed when the vehicle is not in motion.
I would disconnect the large B+ cable from the alternator and take it for a short drive.
I have run into this 3 times that I can remember.

Nice, glad you got it figured out!
When I worked in the junk yard, one of the mechanics was about to install a second transmission over that alternator problem until I finally convinced him to humor me by unplugging the alternator (which was making a terrible howling should).
There was no way that I was going to pull a second transmission out of a wrecked Pacifica for him when I could hear that alternator, lol.