In addition to the above...

Have you got the basic suspension settings correct? Too much sag/too little preload (or a knackered shock spring) at the rear will cause bad headshake, more so at higher speeds where the wind acting on your body transfers more weight to the rear. A lot of high speed wobbles and tank slappers can be sorted by getting more weight over the front, add preload to shock, raise the fork tubes through the yokes 5mm at a time. Too little sag on the forks and too much front rebound damping will have the same effect (although this is less likely in your case, unless that is you've changed things up front (fork spring spacers, more viscous oil, changed the air gap etc.)

__________________