Just saw this post. Glad you upgraded the receiver and used a WDH. I just have 2 questions. Are you using a brake controller? And have you considered air bags for your rear springs?
With the V6 the air bags will help with the ride quality and help level the rear droop even better. They are no replacement for a WDH but they really do help.
And I hope that trailer has brakes built in or you are putting all the brake load on the 4Runner which is a recipe for disaster. Any trailer over 1k should have brakes integrated so you can use a brake controller to neutralize the trailer's weight.
I pull a 3800# trailer using a WDH, Prodigy P2 brake controller and Firestone air bags. Even with the V8 the brake controller makes a huge difference and is by far the biggest safety feature since the mine came with the tow package (transmission cooler, 7 pin connector, and 7300# receiver).
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