Tranmison not shifting - 1998 Ford Windstar

Something wrong with the shift lock system maybe. Safety system that won't allow the gear selector to move from park unless the driver is pushing the brake pedal . Do your brake light's work ? Does your caddy have floor or column shift ?
Automatic Transmission Shift Lock Control Description and Operation (Floor Shift)
The automatic transmission shift lock control system is a safety device that prevents an inadvertent shift out of PARK when the engine is running. The driver must press the brake pedal before moving the shift lever out of the PARK position. The system consists of the following components:
• The automatic transmission shift lock solenoid (serviced as the automatic transmission shift lock actuator), as well as the body control module (BCM) and the engine control module (ECM).The shift lock solenoid is located within the floor shift control assembly with vehicles equipped with floor shift.
• The BCM controls the voltage supply circuit of the shift lock control solenoid. The following conditions must be before the BCM will supply voltage to the shift lock control solenoid.
The BCM controls the voltage to the shift lock control solenoid through the shift lock control solenoid controlled voltage circuit. The following conditions must be met before the BCM will supply voltage to the shift lock solenoid:
• The ignition is in the ON position.
• The ECM sends an input via GMLAN serial data to the BCM indicating the transmission is in the PARK position.
• The BCM determines the brake pedal is applied according the brake pedal position.
Since the shift lock control solenoid is permanently grounded, the BCM supplies voltage to the automatic transmission shift lock control solenoid, mechanically unlocking the shift lever allowing the driver to move the shift lever out of the PARK position as the solenoid energizes. When the brake pedal is not applied, the BCM turns the control voltage output of the shift lock control solenoid OFF, de-energizing the shift lock control solenoid. The de-energized solenoid mechanically locks the shift lever in the PARK position.
During remote start operation, the BCM will de-energize the shift lock control circuit, locking the shift lever in the PARK position.

Most cars now days, passing gear is controlled by the ECM. Look at the throttle body. Two cables going to it? one is for passing gear. Governor in the transmission. Could be the cable or the governor. One cable, it's still the governor, have to drop the pan and replace it or the valve body.

Automatic Transmission Shift Lock Control System
The automatic transmission shift lock control system is a safety device that prevents an inadvertent shift out of PARK when the ignition is ON. The driver must press the brake pedal before moving the shift lever out of the PARK position. The system consists of the following components:
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The automatic transmission shift lock control solenoid

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The automatic transmission shift lock control switch

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The brake to shift relay

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The rear integration module (RIM)

With the ignition in the ON position, battery positive voltage is supplied to the brake to shift relay coil. When the RIM energizes the brake to shift relay, the brake to shift relay switch contacts close suppling battery voltage to the automatic transmission shift lock control switch. The circuit continues through the normally --closed switch to the automatic transmission shift lock control solenoid. The permanently grounded solenoid is energized. This locks the shift linkage in the PARK position. When the driver presses the brake pedal, the contacts in the automatic transmission shift lock control switch open. This causes the automatic transmission shift lock control solenoid to release. This allows the shift lever to move from the PARK position.
Shift Lever Does Not Move with Brake Pedal Depressed ?

Check the ATF level on level ground.

Change transmission filter and fluid if it is over 2 years old.

1

Take the rubber boot off the gear shift inside the vehicle and lift it up, exposing the area where the gear shift and the transmission connect.
2

Look for the shift cable and the shifting arm. A single bolt and nut connect the shift cable and shift arm.

3

Twist the nut connecting the gear shift and the transmission. Use a socket wrench and remove the gear shift. Use an open-end wrench to loosen the nut holding the shift cable and the shift arm together.
4

Slide off the sleeve on the shift cable and loosen the shift cable at the other end with an open-end wrench. Both ends of the shifting cable attach to the shifting arm in different locations.
5

Hitch the new shift cable to the 2 anchor points on the shift arm using the same bolts and nuts. Reconnect the gear shift to the transmission and push the rubber boot back into place around the gear shift inside the car.
6

Test the functionality of the shifting cable by moving the gear shift in and out of gear.

Ok....what do you want us to do about it? Take your car to a mechanic. Unless your just not pressing the clutch....

1. • 1 Take the rubber boot off the gear shift inside the vehicle and lift it up, exposing the area where the gear shift and the transmission connect.
     
   • 2 Look for the shift cable and the shifting arm. A single bolt and nut connect the shift cable and shift arm.
     
   • 3 Twist the nut connecting the gear shift and the transmission. Use a socket wrench and remove the gear shift. Use an open-end wrench to loosen the nut holding the shift cable and the shift arm together.
     
   • 4 Slide off the sleeve on the shift cable and loosen the shift cable at the other end with an open-end wrench. Both ends of the shifting cable attach to the shifting arm in different locations.
     
   • 5 Hitch the new shift cable to the 2 anchor points on the shift arm using the same bolts and nuts. Reconnect the gear shift to the transmission and push the rubber boot back into place around the gear shift inside the car.
     
   • 6 Test the functionality of the shifting cable by moving the gear shift in and out of gear.
     

The front goes to the top of the radiator.

What was the codes exactly that Auto Zone gave you.??? it might just be a simple wire harness that may have gone loose since you did some work on the speed sensor.
Here are the trouble codes for your car.  Good Luck and get the car scanned again for a more define information.

P0751 Shift Solenoid A Performance or Stuck Off

P0752 Shift Solenoid A Stuck On

P0753 Shift Solenoid A Electrical

P0754 Shift Solenoid A Intermittent

P0755 Shift Solenoid B Malfunction

P0756 Shift Solenoid B Performance or Stuck Off

P0757 Shift Solenoid B Stuck On

P0758 Shift Solenoid B Electrical

P0759 Shift Solenoid B Intermittent

P0760 Shift Solenoid C Malfunction

P0761 Shift Solenoid C Performance or Stuck Off

P0762 Shift Solenoid C Stuck On

P0763 Shift Solenoid C Electrical

P0764 Shift Solenoid C Intermittent

P0765 Shift Solenoid D Malfunction

P0766 Shift Solenoid D Performance or Stuck Off

P0767 Shift Solenoid D Stuck On

P0768 Shift Solenoid D Electrical

P0769 Shift Solenoid D Intermittent

P0770 Shift Solenoid E Malfunction

P0771 Shift Solenoid E Performance or Stuck Off

P0772 Shift Solenoid E Stuck On

P0773 Shift Solenoid E Electrical

P0774 Shift Solenoid E Intermittent

P0780 Shift Malfunction

unscrew the strut bolts and the four frame bolts unbolt the axles from the spindles take and unplug all the sensors disconnect the shift linkage cable but be careful with it high cost to replace and it is still a deal item them support the engine with a bar and chain jack up the car put it on jack stands far enough back that it is not on the frame loosen steering knuckle also disconnect power steering line let jack down the frame should fall with the jack remove frame out of the way take out the starter then remove plastic covers unbolt fly wheel bolt then unbolt trans put a jack under the tranny use a pry bar to move the tranny from the engine and lower trans.reverse to reinstall