Volvo Repair: 99 XC70 OBD code intrepretation, shaft speed, output shaft

Question
Roger,
I have the check engine light on so I took the car to AutoZone to have the codes downloaded.  I have code P077 Output Shaft Speed Sensor Circuit-No Signal and P1633 Accelerator pedal position sensor 2 signal too low.  Can you tell me what sensors may be causing these codes?  Note: I recently had the ABS Control Unit repaired which did cause the ABS, SRS, TRACS and Mode W issues to disappear. The gent(VR) that repaired  the ABS Control Unit did indicate that it tested fine prior to returning it to me.  Your opinion regarding this matter is most appreciated.  

Answer
John, I dont know anything about PO codes but these are very vague descriptions.  The pedal sensor is a common issue with these cars.  That has to be purchased from the dealer.  The output shaft deal I have no idea.  I would try your Volvo Shop or dealer and have then scan it properly.  Roger