How many oxygen sensors do you have? I thought the 99+ 4.0's had 4. Are you sure you replaced the correct 02 sensor? P0138 is the bank 1 downstream voltage too high (H02S-12). If you have the 4 sensor system just make sure you are looking at the right one. If you had a scanner that could monitor live data you should be able to see what the sensor is reading when the failure is present. Oxygen sensors normally produce a voltage around 0.2-0.8 volts, the downstream sensors should produce a more steady reading usually around 0.4-0.6v while the upstreams should be constantly switching between 0.2-0.8v when running. I think that code gets triggered when the voltage exceeds approx 1.2v maybe a little less. If you see a voltage higher than 1-1.2 volts then unplug the sensor and see if the voltage goes away, if not then the wiring is shorted to power somewhere, if the excessive voltage goes away after unplugging the sensor then the sensor is faulty.
Does this pic of the exhaust/sensors match what you have? If so the sensor you would be looking for is the #4-1/2 in the diagram.