If it makes anyone feel better this oscillation over certain roads happened with the 1991 and does on the 2008. Weight does help but it's not just cargo but several things I've found.
On the Hilux spring rate and shocks helped and there was a marked difference with WilderNest on vs off.
When I say tuning it's a matter of getting the shocks matched to the springs, in my case it was using the right model OME shock with my heavy packs. Prior to 2008 I had a set of packs that were based on Northwest Offroad with extra leafs. Matched with OME shocks it would bounce. At Rubicon I sheared some of the leafs and used that as an opportunity to get a new set of OME. The ride height didn't change but the ride did. It rode stiff but fairly controlled with the 'Nest off still. When I dropped the ride height I took a leaf out to make them mediums and the ride got worse again.
On the Tacoma I've never quite nailed the ride but stock, stock plus add-a-leaf, OME mediums and now OME mediums with 2 extra add-a-leafs hasn't ever really been perfect but it's not bad now with OME shocks.
So what it boils down to is if you can, use a known, matched setup. Beyond something like OME that has some design intention it's kind of a guess. I had FOX rear shocks that were supposedly tuned to OME but I think I was still oversprung for them. When the OME shocks wear out I'll probably go back to FOX or something to see if I can dial it in but I'm in no hurry at the moment. External adjustments are a real benefit here.
What I *do* notice is my Tacoma has a lot of bounce in the frame. You can tell this when you put your hand between the cab and bed. On the 1991 there was almost no movement on the highway but in the Tacoma it'll squeeze your hand uncomfortably. This is true of Tundras, too. Toyota made them more flexy for soft American butts.
So it's a bit of a crap-shoot trying to tune your suspension by seat of the pants. You'll have a good controlled ride but it'll still feel bouncy and there's really nothing you can do about it. The only ways to really fix it is frame reinforcement. Sliders that are themselves stiff and mount solidly is the simplest way and that has tamed my truck reasonably well.
