This weekend while pulling a bumper hitch trailer w/ 4 horses in central Texas hill country, I noticed my trans temp guage was climbing a little higher than normal (145)... it got up to about 204. When I got to the ranch I let the truck idle while I stalled the horses. Next morning when I checked the guage log for trans temp/pyro/turbo I noticed I reached a max tranny temp of 276! This morning I hooked up to my gooseneck flatbed trailer w/ gross trailer weight of only about 7000 lbs. and drove about 260 miles to East Texas, pretty flat haul, tranny temp stayed about 145---160. When I arrived I let the truck idle while I unloaded the trailer about a half hour. When I got back in and checked the guages the tranny temp was at 252. What's up?

 

Where are you taking your temps, Pan or lines? It doesn't make sense that it would rise with the truck shut down.
I found out this last Summer that once the trans gets hot in 100+ weather it won't cool down idling in neutral, I had to take it for a drive without the trailer to cool it down.

 

The temps are being taken from the pan, and they for sure came up while the truck was at idle. Maybe the sensor is on the blink.

 

I assume you had the tranny in park while at idle. If so, no tranny fluids were circulating thru the cooler. Tranny was soaking up temp. transfer from the engine.

Try leaving the tranny in neutral next time and see if the temps stay down. Supposedly, in neutral the tranny fluids are circulating thru the cooler.