I think the first question you need to answer to yourself is whether the temps you are seeing are because you are doing something different than normal (driving habits, more stop-and-go traffic, bigger load, etc) or if these temps are higher than normal with no other change. If the answer is the latter, then you need to have it looked at. Simply throwing a cooler at it is a band-aid fix for an underlying problem. The only reason to add cooling would be if you are certain everything is solid and you are trying to combat trans temps due to a poor cooling design and/or heavy loads, stop and go, etc. I am not familiar with what is considered "normal" for these trucks (just got my VMax and came from a Dodge diesel) but mine has been running 160 at the highest and typically 130-140 once up to speed and cruising. My Dodge ran around 165 at best and would easily reach 185 empty in stop and go, and would reach 195+ towing at low speed (highest I ever saw was 227 backing a trailer). However, the 48re Dodge trans runs hot anyway compared to some others.

I would guess that you have slippage either in the converter or trans that is heating the fluid higher than it should be.