That's a long grade, but in near-freezing temps I'd be surprised if the trans fluid actually got that hot, unless your cooler isn't functioning properly.

I've had my AT temp light come on while rock-crawling on an uphill trail, and while towing a 1500 lb trailer up a 5-7% grade, but both instances were in the AZ summertime with ambient temps around 100 degrees. I have an auxiliary trans fluid cooler installed in addition to the stock radiator loop.

95 Montero SR
3.8 MIVEC, Advance headers, 2 1/2" exhaust, Magnaflow muffler, OME shocks & rear springs, 2" body lift, 3" tank lift, 4.90s, TRE front locker, factory rear locker, Roger Brown Rock Sliderz, 315/75r16 (35") tires, Sport big brakes