Don't post here often, but wanted to share my findings
I had been looking to get a Scangauge for a while, however the price and styling was a little too out there for my liking, so I came across a Ultra-gauge, which is a credit card sized version basically, but uses a different coding method. There's a lack of info on custom codes for these on the Jeep side, but I took the plunge anyway and ordered it direct from the US.
So with around 4 hours of trial and error in the driveway, I've managed to 'crack' the code and get Transmission temps displayed, and I'm rather pleased with myself. I haven't confirmed these with a dealer computer, but based on the figures I'm getting, they look to be correct. (around 160F when driving)
I haven't managed to get Celcius out of it so far, but knowing the safe range in F is enough for me anyway. A couple of times it's thrown the units out by 1000 on startup, (or my tranny is running at over 1000 degrees) changing page and coming back fixes it.
I've attached pics of the two codes needed.
The ATF ECU code has to be displayed at least once, so as to 'wake up' the Trans ECU for reporting, otherwise the temp is not reported. The ATF ECU keeps reporting 'Err' but the code works anyway. (I assume because the response is 'ON' not a number.) You can put it on the 2nd page and just scroll past it once if you need.
I just took a punt on the gauge being suitable for what I wanted, and great success! Hopefully this may be of some use for someone else out there, and save them hours of confusing hex codes.
(It's for a early KK 3.7L /w 42RLE Trans, I can't guarantee it will work for everyone, but there's always hope)