To set this up in your minds, I just did a trans fluid change about 2 months ago. Torque converter, and trans, forgot to do the cooler.
Last week I hauled my 27' 5th wheel (10K lbs) up to Kernville, CA north of Bakersfield. Make a mistake in navigation and took the 58 from the 14 over to Bakersfield. Should have gone over Walker Pass on the 178. Wound up going up the Kern River canyon. Slow, narrow and windey. Lots of curves up hill probably 30-40 miles. Trans over 220* for long time. After Kernville, road got narrower and steeper for another 30 miles. Tranny ran at 250 for long time. Could not stop, I would have blocked the road. Worried. Added a little fluid at the camp and ran it home, a lot down hill on way home. When got home, noticed engine stumbling when changed gears like torque converter engaging . Talked with my good mech guy and he suggested I take it to Angle's Trans Shop here in Mission Viejo. Talked to Mike the owner and explained the trip, heat issue, and symptoms. He suggested I add some ATF and see how it goes. Added a quart and a half and all's great. He also said if I am towing again that I should change all the fluid this fall.
Happy guy that so far, all seems normal.
Any thoughts are welcome.

 

I would flush the trans now, but that is just me. I'd also add a 6.0 trans cooler after the flush -- it makes a very big difference in minimizing the trans temperatures.

 

To prevent further damage; you need to change the transmission fluid and filter now.
The high temperatures likely burned/depleted the fluid. The shudder you're feeling when the transmission shifts, can be caused by burned or depleted transmission fluid.

 

Do you have a 6.0L cooler in your truck? If not I’d order one, then when you do the installation do a complete flush. It’ll make a huge difference in cooling your transmission system in situations like you just experienced.

 

Woodnthings8 posted / wrote this very helpful article many years ago and I did this (including external large filter) about 8 years ago.

https://docs.google.com/viewer?a=v&p...MGM2YzhiYTliZQ

 

I agree with the others, a flush is in order. Why did you add 1.5 qts of fluid? Was it low? When you got to the steep grades and it hit 250 did you try shifting to 2nd and going slower?

 

Added 1 1/2 quarts; did the level drop that much during the trip, and if so where did it go? I'm hoping you didn't just add fluid ***** nilly as a way to 'fix' something.

 

My dos centavos is drop the pan and inspect the magnet. If it's a big clump of random iron shavings you have a decision to make; risk a flush and hope for the best? Or rebuild.

When I seriously overheated my trans in Idaho (complete noob with truck and 5er, trusted the dash gauge) it looked bad. Trans fluid was a dark smelly brown, complete with shiny glitter like a bass boat. New flush and fluid got me back on the road. That was like 40k ago. Factory Tech valve body, 6.0 cooler and real trans gauge came later when I got home. So moral of the story the trans might be just fine after a flush and fill, so give it a shot. Hold off on the cooler for now. Any leftover fine particles of clutch material and metal shavings may be trapped in it. The small passages of the 6.0 cooler will trap some, and a later high temp use will release it. If trans goes bad a few weeks after new fluid, that shows it was already done for. If it's still going strong I'd get the cooler and a Magnefine filter plumbed before the intake side of it. If there is a lot of material in the pan replace the filter. Otherwise they are pretty much a lifetime item. Not much more than a strainer anyway.

And you bet fluid can be lost; the front trans seal can burp fluid when hot. Classic example is when backing a trailer up a slope, overheats pretty quick. Modern seals are viton to solve the issue.

 

I'm on the mobile site so I can't see your mods if any. I definitely recommend a 6.0 trans cooler. Also.. Are you tuned? Are you running a chip? It would be a good idea not from a power stand point but to have better shift /TC lock up strategy. A quicker /firmer shift and locking the TC sooner and keeping it locked longer will produce less heat.