You may need some help fine tuning the hitch head angle and the leverage arms.
I removed all the steel spring parts front and rear and went with a level ride air suspension on the truck (air bags front and rear with regular shocks). The rear air bags have expansion tanks to help cushion bumps even more. Air shocks do not change the spring ratings or axle ratings and if over inflated can make for an even stiffer ride.
Determine your actual tongue weight with scales. The factory literature tongue weight numbers are usually very low as compared to the reality. The 450 pound number could be closer to 650 or 700 pounds depending on the hitch you are using.
As an example, our 25FB International Serenity has a literature tongue weight of 833 pounds, but with the Hensley hitch head installed at the dealership with full propane tanks, the Shurline scale reported a 1,150 pound tongue weight.
You might find you are overloading the hitch on the Highlander.
Good luck in your research.
WBCCI Life Member 5123, AIR 70341, 4CU, WD9EMC
TV - 2012 Dodge 2500 4x4 Cummins HO, automatic, Centramatics, Kelderman level ride airbag suspension, bed shell
2014 31' Classic model 30 twin beds, 50 amp service, 900 watt solar system, Centramatics, Dill TPMS, disc brakes, 16" tires & wheels