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I'd be a little concerned about the quality of a $3000 Allison transmission.
Look up how to clean your cooling stack. Maybe the cooler is plugged with bugs and debris and such blocking air flow. It's an issue on anything, but I think I've heard these trucks are more prone to this becoming an issue. Stuff accumulates between the cooler and the radiator and blocks air flow. Trans fluid goes through the radiator part of the radiator as well in that truck.
What are your guises thoughts on a rebuilt tranny?
Here is one that is beefed up with a bigger oil pan, and that price included the 2,100 core charge. https://www.thoroughbreddiesel.com/1064734bm/