That sounds like my problem, lumpy and rumbly! I'm only on 69k and looking underneath today you'd think the car was only 3 years old. We checked all joints and bushes with a pry bar, checked bolts etc but all is solid. The rear drop links were very easy to swivel in their joints which surprised me as I expected some resistance but no other movement. I'm reluctant to take it to Lexus, especially as the closest is over an hour away. Generally it's fine on most surfaces, just annoying over small bumps. I don't mind spending to get it sorted, it would just be nice to know what it is. Even my wife has noticed it, and that's unusual!!