As my 2000 RX300 changes from 1st to 2nd gear, I get a very light chatter (vibration) right at 18-20 mph that goes away once it shifts into 2nd gear. It occurs just before the shift and only lasts for a couple of seconds. The only time it's noticeable is when I put the engine under a bit of strain...if I just ease it up to gear change, it really isn't noticeable. If I really step on it, it's enough to make a plastic cup in cupholder rattle, which is how I noticed it the first time. Vehicle has 79K miles and otherwise accelerates smoothly.

 

Could be some suspension piece loose as the change from one gear to the next, whether abrupt or not, can have enough force to cause that.

What happens is that once the vehicle gets enough speed (or forward force so to speak), whatever suspension or loose piece is being pushed by inertia so you won't hear a peep, but come low mph and it will chatter. I did a thread just recently about some culprits, https://www.clublexus.com/forums/sho...d.php?t=314746

 

One other thing I should mention, if you have AWD, there can be a noticeable abrupt shift between gears when the gear oil gets old (for transfer case and rear differential). I noticed it more between 2nd and 3rd until I switched out the gear fluid, but your situation sounds more suspension related.

 

Sounds to me like clutch chatter. I.e if you put dexron fluid into a chrysler trans. it would do that. They are supposed to take atf plus 3,4 fluid. Ours are type-iv fluid. Did you change your fluid lately? if so,dont use anything but type iv fluid. And don`t top off with any other fluid either. Btw Lexmex,I did some research,and found out that toyota atf type-iv trans. fluid is synthetic. And supposed to be very good fluid.