I have a new RAV4 Hybrid. There are many cautions about towing in my manual but they do not necessarily apply to the engine. As a matter of fact, the hybrid using the engine and the electric motors produce more HP and torque than an the engine alone.
The problem, at least with mine, is that the TV itself is not robust enough to tow much weight. In my case the maximum weight I can tow is 1500
pounds. I am pretty sure yours will be higher but I would check. There should be a sticker on the door jamb listing the combined weight rating for towing. Check with your Toyota dealer to get the accurate numbers for you specific vehicle. Ask THEM about the issues with towing for your particular vehicle.
All that said, I towed with a 4Runner years ago. It had a V8 and was built on a Tacoma frame, not the unibody of a Highlander. I had a 5000, or was it even more?, tow rating.