I'm here in phoenix too. I use this cooler: Hayden-Automotive-679-Rapid-Cool-Transmission
Temperatures measured at my remote filter on the hot side of the cooler range from 145 to 165 degrees in the summer. Or winter... it doesn't vary much.
I think the highest temp I've seen after installing this cooler is 195 degrees - that was low speed wheeling in 4HI. (good use of the torque converter)
Are you bypassing the radiator altogether with this cooler?
I've changed my mind and have decided to forego a new radiator at this time and add an external cooler instead. I believe this is the cooler I'm going to go with. I know the 678 is the most common Hayden recommended, but I like the bigger 679.
Thanks