There is a H block type thermostat (I assume it's a thermostat) that can malfunction and not cycle the trans fluid to the trans cooler. Somebody with more insight should chime in soon. In the mean time, search the forum for transmission H block. I know I've read about it before.
2013 2500 CCLB 4x4 Laramie
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2013 2500 CCLB 4x4 Laramie
Sent from my iPad using Tapatalk