'14 IS350 F Sport AWD - Rear Wheel Issue

[S19]

Hi all. Wheel/handling issue.

I purchased a set of the Michelin Pilot Sport A/S 3+ upon needing to switch out the original dealership tires. I had these put on and then I went to a Lexus dealership and got a full alignment. That was six months ago and the car has been "fishtailing" ever since. I hit a bump and the back end kicks for a moment. The worst is driving in the rain. The back end snakes all over and it feels deadly. I had the dealership re-check the alignment and they say it's perfect. However, my back tires are wearing much quicker than they should. 

Anyone have any ideas what the issue may be? Thanks! ~S