Strange problem with my 2000 4WD model; this happened last year when it got cold as well but now it's getting worse. Seems like when the first driven from cold (but not every time), at speeds above about 25 mph the car gets a terrible vibration that gets worse as speeds increase; like a jackhammer sort of shaking. If I come to a complete stop and then drive away, the problem disappears and almost always never returns as long as the car is warm.

When the vibration happens I have tried shifting into neutral or lower gears to see if it's transmission related and I don't get any change in the condition. What could it be? CV joints? Something in the 4WD system? I had new brake pads and calipers a while back so I don't think it's a sticking caliper (vibration occured before and after the new calipers)

 

Find an empty parking lot, early Sunday mornings are good for that, and run some figure 8's making the turns as tightly as you can. With the windows down or with an observer you should be able to rule out the CV joints if you don't hear clicks.

In 2001 Lexus stopped offering LSD on AWD. But your 2000 may have a limited slip differential. If so, have you changed the gear oil? and if so did you add the correct LSD additive? The figure 8s will also expose grabby LSD clutches as you transition from straight to max turn and back. Too much LSD additive and the clutches won't engage and you're back to an open diff. Too little or no LSD additive and the clutches stick and won't disengage.

Are your tires flat spotting on cold cement over night? Are they at proper inflation?

Check motor mounts, trans, mounts, and diff mounts.

Check the dampener part of your struts. If backing out of your drive shifts a damaged strut into a sort of locked position then as you drive off it will transfer a lot of the road imperfections directly to the frame (unit body). It could be that the stop going forward shifts it back.

Are the tires wearing evenly? Does the car pull to one side? When did you last rotate the tires? These are non-directional radials so use the X pattern.

 

thanks for all of the information! I will try these things before I take it in so at least I have a better sense of what it might be.