Full lockup TC for 8.1L/4L85E?

'01 Yukon XL 2500 4WD
From what I recall reading my TC doesn't fully lock up; something to do with the friction material. Does anyone have experience with either mods to stock TC or an aftermarket that is more towing/mpg oriented instead of high stall? This thing is still my daily driver so I need all the mpg help I can get. Plus I tow a 10k lb boat so no high stall for me.
I also need to get the ECU done so it locks up at a slower speed and maybe lock up in 3rd instead of just OD.

Thanks,
Michael

Your TC lockup settings are in the PCM. You have a custom tune yet?