I should have noted this:
The Yukon has been very well maintained at a local GMC Dealership. Every 30k miles up until about 120k the transmission fluid/transfer case/diffs were flushed, so it's definitely not a situation where it was neglected.
The tranny fluid is bright red, looks brand new, and there's not even a hint of brown really.
The stock transmission cooler is fairly large, not much smaller than the one sent in the link above however I'm sure the tru cool does a better job, it looks a little thicker anyway, I'd be surprised if a new slightly larger cooler makes a 30 degree swing on temps, but I could be wrong. There's only so much fluid running through the cooler at any given time and it takes a while to cycle it through.
I know I said that the whine gets louder with temp, however it doesn't really go away when the temps are in completely normal operating range. 180 - 200 degrees for a transmission isn't crazy hot IMO, and especially not when towing.
@APT - Yeah the main reason I was looking at the ecoboost, or an old Excursion was because of the turbos and the elevation I tow at. We plan to do a lot of Colorado camping because we live here and it's "easy"...easy until we have to go up passes and back down them with a trailer. For resale and longevity I'm not sure how the Ecoboost will hold up, I haven't seen a long enough track record to tell how durable they will be. I think the next best towing SUV would probably be the Toyota with the 5.7, I've owned a Tundra previously and I know that engine/transmission are up to the task, I just need to also haul 4 kids and a wife :).
