It sounds like a wiring problem from your computer to the sensor. I am assuming that you used a new GM Coolant Sensor because there is an updated version of that that comes with a newly designed connector and wiring. See if you can have someone scan the coolant sensor data on your truck to see what the computer is 'seeing' when the sensor is plugged in and when it is unplugged.
here are some shops, they should only charge about 1/2 hour to look at the coolant sensor data
http://repairpal.com/directory?address=44634&car_brand_na...