Judging from the fore-aft brake & acceleration diving on my SR5, and the nice way it rides, it's safe to say it's softly sprung.
My 2 previous rides were Pathfinders, which were like a 4R; great SUV's, but softly sprung. Even with 2 adults in the back the PF sagged and rode 'bouncy'. When I towed my boat, it was awful until I installed Timbrens.
I imagine the TE and LTD suspensions deal with this issue better than the SR.
I no longer tow my boat, but if I towed anything with an SR5, I'd have the Airlifts or Timbrens on it in a heartbeat, the Airlifts being the probable better choice. They're both basic, low cost solutions.