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The wheel speed sensor is located in the wheel bearing. This sensor provides input to the abs computer. The computer can use this information to determine wheel speed and calculate wheel slippage for traction control or abs functions and can also calculate tire pressure. The wheel bearing is likely bad and causing the problem. If you had no problem like this previous to the transmission repair then it is possble the wiring at the sensor was damaged during the repair, as work is done in this area to drop the front suspension subframe in order to get the transmission out and back in.
This repair normally takes about an hour to complete. If you are an experienced do it yourselfer with a decent amount of tools then you could tackle it yourself. It is best to have an air compressor and an impact gun to remove the axle retaining nut.
If you have a garage do this repair, it will usually cost around $300-350 including parts and labor.
Your code reader only reads powertrain codes, it takes an advanced scan tool to read abs codes.
If you have an ohmmeter, disconnect the electrical connector going into the backside of the bearing (behind brake rotor) and measure resistance across the two terminals going into the bearing. You should have approximately 1100 ohms resistance if the sensor is okay.