When should you perform a transmission fluid replacement on the 2004 toyota highlander V6?

If you go to

and click on the owners links, you can get online access to the maintenance schedules for your vehicle.

Prior to transmission fluid replacement, the site lists replacement of the rear differential fluid of 4WD models.

Automatic transmission fluid (ATF) replacement is at 60,000 miles if you tow, otherwise, ATF inspection shows up periodically. If the ATF is being inspected, then presumably, an inspection would indicate when replacement is necessary. I personally would just follow the severe or towing intervals.