The motor/generator sits in place of the transmission in my Prius so I thought you were full of beans when you said the Highlander has a motor on the rear wheels. It appears it's in place of the differential in the hybrid Highlander. Never knew that. As much as I like 52+mpg city with my hybrid while DW drives my F250 for her short commute, I too don't recommend towing anything with a hybrid. Not so much the electric motor/generator but the digital variable frequency drive can overheat and fail.
http://en.wikipedia.org/wiki/Hybrid_Synergy_Drivehttp://en.wikipedia.org/wiki/Hybrid_Synergy_Drive