I have a similar situation with a 2006 Honda Pilot 4WD rated for towing 3500 lbs. I pull a Lance 1575 (a very nice trailer--check them out) with a GVWR of 3500 lbs, and fully loaded it is just shy of that.
The Pilot has adequate power on flat and rolling terrain, and even does OK on moderate grades if I let the rpm climb into the power curve. Definitely needs more power on steep grades. It is a 5-speed, and really needs one more gear. There is too big of a gap between 2nd and 3rd. Paddle shifters would be great too, as it would make it a lot easier to downshift when descending.
The suspension needed help. Timbrens (rubber overload springs) were installed in the rear, and a weight distribution hitch was necessary to restore the weight that was transferred from the front axle to the rear axle. I use the Andersen WDH and really like it.
Overall, the Pilot seems to be at 100% of its capability. I would like a more robust SUV that has more towing capability but haven't found anything that I like. So many SUV's seem to fall short on the "utility" part of the equation. The ones that are capable get terrible gas mileage. Sometime in the next year or two I will have to decide...
I think you would have a similar experience with a Highlander pulling a 3500 lb. trailer.
Bruce