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..isn't the "cooler" found inside a rad actually a heater for cold weather operation?
...necessary for winter operation.

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No it is a cooler. Don't try to run without it unless you have an external cooler installed. Transmissions have been lost because of this type of thinking. In fact OEM external coolers were routed through both.

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Me thinks he is correct, heater, not cooler, the oil to water trans cooler in the stock radiators is a heater for the transmissions, not a cooler per se I've ran a really loose(5600 flash stall) race converter on the street with a small external tranny cooler and the tranny temps never got above 180 F after driving it for 20 + minutes in slow traffic in the summer here I had my first tranny temp. gauge on my 1998 1/2 Ram 2500 CTD 24 valve Pickup, the trans . temps never got above 120 F in the summer or winter with the stock internal cooler and the stock added on external trailer towing package cooler when running empty in the Mojave desert It would run 160 to 200 F in the summer when towing my race car trailer, (24 Ft Pace Shadow, 8650 lbs on the axles) enclosed with the 3450 lb car in it Most ATF tranny fluids need to be operated at or above 160 F for some of the additives to work properly OP, use the in radaitor only option and see what the fluid color does