Here is the full run down from the Manual on P0136 :
P0136 - HO2S Sensor Circuit Malfunction (HO2S-12)
The downstream HO2S sensor(s) are continuously checked for maximum and minimum voltages. The test fails when the voltages fail to meet the calibrated limits.
Pinched, shorted, and corroded wiring and pins.
Crossed sensor wires.
Contaminated or damaged sensor.
I only found 1 other post where a memeber had this error.
O2 Sensors Question
Seems his soultion was a replacement of the ( correct ) O2 sensor. While this may or may not be the same as yours, taking a look at HO2S-12 to see if the wiring is damaged, or if the pin connector is not crapped up might be a place to start.
All other searches on P0136 had no additional information.
Have they been replaced at all ? The only reason I ask is if they are the origional, maybe the pins are corroded in the 4 pin connecotr ( truck harness to O2 hharness ) and it might be a case of corrocsion causing a high resistance connection, causing a voltage drop, which causes the code. ?? Just a guess.
Good luck with this one, post back when you find the answer for the next guy.