I joined up a while back and have been reading this forum. I have a '05 Envoy SLT 2wd. It started throwing a P0128 code so I studied this site and replaced my thermostat and coolant temp sensor. That fixed my problem as well as increased my gas mileage back up by 2~3 mpg (around 22-23 highway). Just wanted to say thanks for all of you guys sharing your experiences and information. Maybe before long I can help somebody with a problem.
My Envoy has around 115K on it now. I just put new rear shocks (Bilstein HDs) on it and plan to change the fronts as well (got the Bilstein info here too). I also plan to clean the throttle body and change spark plugs.
Thanks again everybody for the help so far!