When I was experiencing this, the best I could describe it was like the suspension and drivetrain shuddered or lurched. I could feel it profoundly in the steering wheel, but everybody in the car could feel it to some degree.
I felt this when I first bought my truck and it was on crappy, discount brand tires. I immediately had them replaced with BFG 285/55/20s, but keep in mind that at that size, they are only available as D-rated. The recommended PSI should be 43 if I remember correctly.
No matter, because that never actually fixed the problem. If anything, it made it worse. Maybe this was due to the extra unsprung weight.
Again, having now been driving on the 17s with 285/75/17 E rated BFGs, it finally rides how I think a Lexus should ride. Still rides like a body on frame truck, but much more comfortable and no more crazy jolts.