Originally Posted by Bossman

According to a 4th Gen 4Runner Scheduled Maintenance Guide, the transmission fluid should be first inspected at 30,000 miles, then again at 60,000 miles.

If you use your 4Runner for towing, under the Special Operating Conditions it is recommended to replace the transmission fluid at 60,000 miles.

Next inspection of the transmission fluid is at 90,000 miles, then again at 120,000 miles. Transmission fluid replacement (under SOC) is recommended at 120,000 miles.

Here is a link to a video explaining when you should change your transmission fluid, answering whether or not changing your transmission fluid could damage your transmission, and whether or not to change or flush the transmission fluid.

YouTube

And this link to a video outlining the steps to actually change (not flush) the transmission fluid.

YouTube