my 2013 XTS has always had a slight shimmy/shake at cruising speeds (of course it would be) i have paid for alignments, several sets of new tires, and COUNTLESS returns for rebalancing only to help somewhat with the problem. now i'm wondering if it's the tires or the magnetic suspension as it will be smooth as glass, then go to shake, then smooth back out. someone just told me today that their new Escalade had this issue and it was "something" with the suspension but they don't remember what and that the dealership repaired it first time. anyone have this issue and if so, please tell me what it was. it's a shame my 2007 Honda Civic with over 220K rides smoother than my Caddy

 

Yes, mine does the exact same thing. I hit around 65-70MPH and I get a mild vibration. Then it smooths out. I've tried everything too. Phantom vibration. I'm not going to bother taking it to the dealership. They will only charge me $250 only to tell me they have no idea what it is. I'd rather spend that on the lottery.

 

been there
you may have a slight defect/bend in a wheel
it may be too slight to find on the high-speed balancer
i'll assume you don't have a fifth wheel; put the spare on until the shim goes away [be mindful of speed and air pressure of the spare]
good luck