Speed sensor light flashing ,awd drive seems to not shift properly, car drives erratic as if the shift cannot decide what wheel should or shouldn't be getting power ,this is at lower speeds,once I get to about 80 k tranny shifts to 5th or 6th it drives properly ,but if Ishift to4wd it goes erratic again

Look at the transmission control module. Sounds like a massive wiring harness problem too.

Yes and no. Just depends on what it is called. ABS sensor and wheel speed sensor are the same thing and function the same way. That is what is going to be bad and there is one on each wheel. Put the car on a computer at the mechanics shop and they can tell you which wheel sensor is going out.

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Sounds like you have an electronic shift diff to put it in 2 or 4 wheel drive. You could have a bad position sensor, or shift motor/linkage problem. Unfortunately, unless you have the diagnostic tool to see which is the problem, it is best to take it too a good tranny shop.

No 4X4 switch on the instrument panel ?
Automatic Transfer Case
The single speed automatic transfer case system shifts automatically from rear wheel drive to four wheel drive when the transfer case shift control module (TCSCM) senses the difference in rotation speed of the front propeller shaft and the rear propeller shaft . Two vehicle speed sensors, one located at the rear output shaft area of the transfer case, and the other, located at the front output shaft area of the transfer case, send front and rear propeller shaft rotation speed information to the TCSCM.
When the front and rear output shaft rotations remain the same, the TCSCM sends pulse width modulation signals to the encoder/motor to keep the transfer case in rear wheel drive. When the front and rear output shafts rotations are different, the shift control module sends pulse width modulation signals to the encoder/motor to engage the transfer case into four wheel drive.
The system will automatically transfer a percentage of torque to either the front or rear axles as required to stop all wheel slip. When the front and rear output shafts rotations return to the same speed rotation, the shift control module sends pulse width modulation signals to the encoder/motor to position the transfer case back into rear wheel drive.
When the transfer case is engaged into four wheel drive, a lamp indicator on the instrument cluster will light. The automatic transfer case system is completely automated, no transfer case control button switching system is required.

Do you know what DTC'S - diagnostic trouble code's are ?
Auto part stores can't read these , your best bet take it to a qualified repair shop . One that has the knowledge an diagnostic equipment .
DTC C0300 Rear Speed Sensor Malfunction
DTC C0305 Front Speed Sensor Malfunction
DTC C0308 Motor A/B Circuit Low
DTC C0309 Motor A/B Circuit High
DTC C0310 Motor A/B Circuit Open
DTC C0315 Motor Ground Circuit Open
DTC C0374 General System Malfunction
DTC C0376 Front/Rear Shaft Speed Mismatch
DTC C0550 ECU Malfunction
DTC C0611 VIN Information Error
DTC C0895 Device Voltage Malfunction
Without proper factory service an repair info. an diagnostic tools

Sounds like you might have a wheel speed sensor that has failed. If you have the system scanned it should indicate where the fault lies.

There is a small difference in what you are saying here. "AWD" stands for All Wheel Drive. 1999 Ford Expedition is equipped with (A4WD) which stands for Automatic 4-Wheel Drive. There is a big difference between these two systems. The AWD has no driver controls and drives all of the wheels all of the time. the A4WD system ia capable of shifting in and out of 4WD automatically as needed, depending on wheel speed sensor data that indicates wheel slippage. The A4WD also has a 4WD Mode switch mounted on the instruent panel to allow the driver to override how the system operates.

FIRST, check to make sure that the 4WD Mode switch is in the "AUTO" position. If the switch is in the correct position, then you will need to scan your transmission and transfer case control modules and perform visual an functional inspection of the system to pinpoint the cause of the malfunction. It could be anything as simple as a defective mode switch to a major problem like bent/broken shifting forks inside the transfer case, requiring complete overhaul. Proper diagnosis will be required.

check your wheel speed sensors. and vehicle speed sensor on the transmission

drive 4 uses it to sense slip in the front/rear wheels.

check engine light on because it needs to know vehicle speed to shift properly.

no speedometer because it turns the voltage into a signal to the gauge

i had a customer with a 2004 gmc sierra 2500hd 2wd that stated that the srevice brake system light came on while driving. the transmission would shift hard and erratic plus the speedometer was erratic. the check engine light came on with two codes p0502 and p0503(vehicle speed sensor A low input and vehicle speed sensor A intermittent/erratic. i replaced both vehicle speed sensors mounted on the driver's side of the transmission. both of the speed sensors were the same. test drove the truck after and all issue were solved.

Yes if the vehicle speed sensor is bad than the computer can what speed you are going .SO the shifting comes from the throttle position sensor instead and trans shifting will be erratic :)

you have to take to dealership.they have to scan it.you could have a bad wheel speed sensor causing your AWD not to work right.your wheel speed sensor send information to transfercase control module to engage shift motors.