That sounds like real, legit headshake. A bad component will usually not appear...then go away. Usually a bad part might get worse, at speed, but if you pay attention, it will show itself at other speeds.
Now the thought is why. If you are taking these bikes up to speed, you have to know what condition the bearings are in . Wheel, swingarm, shock linkage and steering head. If a person cant do it themselves, pay someone to do it. Not doing/knowing and just hoping is foolish, stupid, and dangerous. Not directed at you. That goes for all of us.
Raising the back end, with a 190/55, tpis a great mod. However, doing so sharpens geometry and cuts into the margin of ...not safety so much, but call it set up error. Sharper geometry relies on the suspension to do its job properly, and set for you. The tire made a huge change in height. Your new taller tire, with its new, deep tread, raised the back end by at least 8mm, if not more.
The best idea would be to set up the suspension properly. Set the front and rear rider sag. If the back end is too stiff, or does not sag enough, it can cause this to occur.
A quick thing to try would be to lower the back and just leave the front as-is. This will change something. It should make it better. If it makes it worse, thats fine to, but be ready. You know what not to do. As you add preload and make the back end more stiff, you usually end up needing to add some more rebound damping in to stay as you were. Not much, try a 1/4 turn. Then reduce that 1/4 to 1/8. Raising both ends will do pretty much nothing.
A damper is a great mod, and everyone should have one. They are awesome. Much better to be able to use them rather than to "need" them. Adding one now is a band aid over something else. Hundreds of these ninjas have made this change with no issues, so its better to find out whats up with yours.