[rickst29]

This information applies to recent 4Runner, Sequoia, and GX-470 vehicles, as well as the equivalent pickups models with 5-speed automatics.
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You need to watch the tach and learn to identify when the torque converter has unlocked itself. When this happens, the torque converter will churn the ATF, causing the tranny temp to rise. If allowed to continue, this can overheat and damage the tranny.

During the first second (or two) after your Tranny has changed gears, you SHOULD see the RPMs drop off by about 150-200 (while holding constant speed). This occurs when your torque converter "locks up", which it does to improve efficiency and keep the ATF temperature down. When the converter is "locked", there is much less stress on the the ATF, because it isn't being called upon to do as much of the energy transfer anymore.

A sudden rpm jump of 200-300 indicates the converter has unlocked. This will happen when you need more torque to maintain speed, such as when climbing a grade or towing into a headwind. If the converter stays unlocked for any long period (over a minute or two), manually shift down (from overdrive to 4th, or from 4th to 3rd), which will allow the engine speed to rise and the converter to lock up again.

Learn to recognize lock and unlock, and adjust your driving (with and without the Trailer) so that you're driving locked almost all the time, with only a few moments of "unlocked" time when you (or the vehicle itself) actually shifts. BTW, running "locked" in 3rd at higher RPMs gets BETTER gas mileage than running "unlocked" in 4th-- when unlocked, a lot of the engine torque is wasted, generating heat in the ATF fluid instead of driving the tires. "Locked" 4th gets better MPG than "unlocked" 5th for the same reason.
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Of course, it's a good idea to downshift ahead of time when you see a hill ahead that you KNOW the vehicle won't be able to handle without unlocking (if you stay in your current gear). Do your downshift shortly before you start climbing the grade.
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Also, of course, learn to manually downshift on long downgrades to avoid brake fade. The rule of thumb is to choose a lower gear that'll let you descend the grade while maintaining your speed without using the brakes. NEVER coast in neutral with a trailer!