You say temp got "almost to the high mark", so Im assuming you have an analog gauge? Do you know what the actual temp was, or just "near the high mark"?
Towing my TT with my Sierra 1500, at times, my digital Tranny Temp registered 210 (think that's the highest it has gotten). Most times it's high 190's to low 200's (less than 210). That is towing with tow mode engaged. Haven't had to drop down to lower gear manually (yet). Normal operating temp around town, not towing, is up to mid 190's in hot weather.
Depending on what the actual temp is when it is "registering near the high mark", you still may be okay. But, if shifting to 4th or 3rd results in lower temp, by all means, do so.
Mike