Transmission not shifting at thirty to thirty five mph

Hello, when i start driving my car, when I reach about thirty to thirty five mph, I have noticed it seems like it does not want to shift gears. This has never happened before. I always take my car to Wal-Mart for an oil change, and from my understanding they are suppose to make sure that all fluids are filled and accurate. Any suggestions on what this issue is? I took it to Auto Zone, and they ran a quick test. Definition: Code P0733 indicates an incorrect ratio for Gear 3 for a predetermined period of time. The results from the car Error Codes Read: P0733 Gear 3 incorrect ratio (transmission shift solenoid), P0882 TCM power input signal low (transmission solenoid/pressure switch). Also, the Auto Zone worker suggested maybe purchasing a sensor (transmission oil pressure) Duralast, part number SU8494. If this information helps, I appreciate you in advance for the feedback! My car has 120,000 miles. My car has an automatic transmission.