2002 Toyota Sienna clunk noise

When the van is rolling at speed, between 40-50 mph, with slight varying pressure on the accelerator pedal, letting off it or applying light pressure to it there is a slight feel of a lag and a clunk. Have replaced transmission mounts, various sensors and the problem persists. Looking on line for a direction to try. Latest we have come up with is possibly worn out bushings. Was wondering if someone knows of a specific problem with this specific symptom. Would greatly appreciate any advice. A more specific answer of a educated answer I would be willing to donate for the help.

Thank you,

Steve Shock