It comforts me that i'm not the only one strugglin at D2/D3 :) Clearly I'm not a champion in playing HS but it was weird that my winrate dropped from 62/63 % to nearly 50% in that range. I checked HSreplay, Vicious Syndacate but one thing it the deck, another thing is to make right play at the right time. And with this close meta a wrong choice (Rotnest Drake or Glaivebound Adept with a suboptimal target) can cost you a lose.
It is not weird to see your winrate drop when you are nearing Legend. It is not weird at all and in fact it is too be EXPECTED!. You are playing BETTER players. I typically get to D5 with around a 70-90% winrate and then it drops off SIGNIFICANTLY.
Just stick with 1 deck and up your playing time. But don't play if you're tilted as you will most likely misplay more often. You also need to be very critical of your plays and really look into whether you are making any significant misplays or incorrect mulligan choices. Good luck.