Toyota Repair: 4runner, automatic tranny, drain intervals

Question
i have  a 95 4runner 4wd automatic tranny. i just bought it used. i was pulling a trailer with it the other day and the a/t transmission fluid temp light came on. is it equipped with a transmission fluid cooler or something? i just need to know why it might be overheating. the trailer was empty so it wasnt alot of weight.  any help would be appreciated. thanks.

Answer
Old fluid, low fluid.
All semi-desint A/T's have a stock cooler. Normally located in the radiator.

And they are not big enough for the drain intervals people use in the real world.
Flush the fluid & Add a descent auxillary cooler after the stock cooler.