I have a 4X2 Crew Max 5.7L with TRD offroad package. I notice the bed bounce even at all speeds. To me, it is more of a rear end vibration or resonance. My last two vehicles were a 2003 Tacoma with TRD and a 2005 Tundra Double Cab without TRD. My 2005 Tundra road like a Cadillac. It was fantastic. My 2007 CrewMax is definitely a rough ride, but it is only noticeable when the rear wheels hit a bump. It is very strange to me. The rear end just vibrates like crazy after hitting a bump. When people ride in the back seat, I can hear the vibration in their voice. Kinda funny, actually.
In my opinion, this is just a case of having a truck with high payload and towing capacity. In order to get that high payload and towing capacity, they've gotta make the thing strong, rigid, and use big suspension. So, it makes for a rougher ride. If the truck was not a lease, I would get a different suspension put in. Maybe something adjustable. However, I'll live with it for a few years and then get the next generation, when they've perfected it a bit.
By the way, my BakFlip cover is being delivered on Monday. If it changes anything, I'll update the thread. However, I highly doubt that it will fix the problem.
No matter what, I love this truck!