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On my TTSE, whenever it happens it is just an indication that it is time to service the tires with air and rotate.
I notice that it will go along and a few seconds later it will be gone. I believe (at least in my case) that it has something to do with turns on the highway and I believe it to be a slight imbalance that at certain times has the imbalance counteracting against itself and other times it is in sync with the opposite front tire.
Getting your wheels rebalanced has not seemed to be the answer for others for I believe that the slight out of balance situation is within the specified tolerance (or even the practical tolerance)[Ain't nothing in this world is perfect] but when the weight align you will get shimmy and after a slow turn in either direction it will go away.
Like I said, to make mine go away for a while, I find a tire rotation and air pressure check will do it for about three four thousand miles. And I do that at the first sign of shimmy. It has always worked.
A further assumption on my part is that the slight out of balance situation is aggravated by, or in conjunction with, uneven wear pattern on the AT's. Thus my theory and correction by rotation.
TRD alloy wheels and BFG AT tires as the vehicle was delivered. TTSE Stock, as delivered, no lift, no aftermarket parts.
If you let it go on and on without correction, parts are going to wear, get loose, and it will become more pronounced and readily returning.