All V8 4th gen 'Runners have a factory-installed tranny cooler. Open the hood and look for the smallish radiator on the right side in front of the main radiator.
I've towed my 4,000 lb trailer for something over 30K miles, first with my '98 'Runner and presently with my '03. My tranny overheat light has never illuminated and neither truck has shown any other signs of heat stress. I've towed through mountains and in summer heat with the A/C running with no problems.
I've never installed an aftermarket tranny cooler and nothing in my experience suggests one is needed. Toyota builds good, rugged, well-engineered stuff. If you keep the load on your truck within Toyota's published limits, you shouldn't have problems.
Good luck.
-Karl
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