Camry porpoises with passengers in back seat

[carpen]

I have 2008 LE V6 and I notice that whenever I have 2 passengers in the back seat the car porpoises over speed bumps and at highway speeds and over rolling terrain. I get sea sick!!! Did Toyota miss the set up on the rear suspension? I also notice a thumping noise coming from the back when accelerating hard from a stop – could be a sway bar issue. It rides fine with 2 people in the front.
Any insights would be appreciated.
Definition of porpoises:
1: to leap or plunge like a porpoise <penguins…porpoise out of the water
2: to rise and fall repeatedly

[hans747]

the clunking sound makes me think you might have a swaybar link or something that's loose or broken. Take it to the dealer and ave them put it up on the lift. that could be a rather unsafe problem is you're driving a lot of highway.

[GT-R]

Your struts are blown.

[thrawnis]

If this is an easy problem to replicate, take it to the dealership and show them the problem. When I've been loaned a couple different Gen6 LEs, I've never noticed this problem even with 4 people in the car...