1998 LEXUS LS400, ~130K MILES, TOWN DRIVING, AUTO SHIFT.
~35MPH DEVELOPS FINE VIBRATION WITH ACCOMPANYING HUM. BOTH INCREASE WHEN SPEED UP. HUM DEVELOPS A HARMONIC AND SHAKING INCREASES TO ALMOST A SHUTTER.
DECREASES OR STOPS WHEN RELEASE ACCELERATOR. COULD NOT TRY NEUTRAL AS OWNER/SPOUSE DRIVING.
i’m only shooting in the dark but, i would start with the driveshaft it has a dampner on the back of it, or it could be a tire/wheel out of balance. i changed my mind take a good look at the tiresfor uneven wear.
This could be a case of torque converter shudder.
I would suggest a visit to an independent transmission shop for examination.
Whatever you do, do NOT go to a chain operation like AAMCO, Lee Myles, Cottman, Mr. Transmission, and similar charlatan shops.
On a '98 this could be any one of 100 things…or all 100.
Have someone check the chassis, the engine operation, the engine mounts, the struts, and "last but not least) the tires & wheels. It could be a bad TC, worn bearings, poorly worn tires, a bad engine mount, poor engine operation (like bad plugs or a misfiring injector), bad tie rod ends, a bent rim, or any of a list of other causes.