This is the reply I received from Sinister Performance.
Cameron, I own a 2004 TrailBlazer with the same engine and transmission your Envoy has. These trucks don’t have a separate TCM – they only have a PCM (that controls both the engine and transmission in these applications).
There is no way to program the PCM for a tow/haul switch, unfortunately. I’ve looked into this for my Trailblazer and there is just no way to do it as the tow/haul mode was not a factory option for these vehicles.
It is a good idea to put the shifter into “3” range so the transmission won’t be able to shift into 4th gear while towing.
I can custom tune your PCM to adjust the transmission shift points and firmness settings (as well as increase line pressure) if you so desire. But these changes will impact all aspects of driving as there is no tow/haul mode switch option.
Good call on the second PCM Mooseman. That's a great idea. Depending on price I might do that. Otherwise just keeping it in 3rd seems to be working just fine. Thanks for all the input.
Ryan Gick
Sinister Performance, LLC.
Waterloo, Indiana USA