Xr100 rear vibration

I know the xr100 is a rattle can, but after my last race I notice a wierd vibration coming from the back end after I enter a hard braking zone. Hard braking meaning downshifts+rear sliding+front brake all at once. The rear will vibrate like crazy then settle down maybe after a couple turns. Could this be common or maybe wheel bearings? Maybe just the rear drum mis-aligning and then centering itself again? What you spec 100 racers think

 

Spokes?

 

do you hear a "clunk" when you grab the rear wheel and push/pull it to either side?

 

Spokes are tight