The variable vane technology was intoduced into the DuraMax with the LLY engine, introduced mid-year in 2004. These vehicles are referred to as "VIN 2" or 2004 1/2 trucks. I tow the mountains here in Colorado, and have had no issues using the Allison stock with tow/haul. Only mod I did was put on a deep Ally pan and switch to synthetic fluid.
