Fluttery Rebas - causes/fixes

I have some Rebas.  Probably got 2000km or so on them of a fairly solidly built rider.  

They are properly juddery to the point I can see the tips of the legs visibly fluttering under braking. Feels like half of them is loose inside. They're nice and smooth when just riding along until you really hammer into something.

Other than wear on the bushings what else might be causing my judder.  

Would a worn brake disc potentially cause this before I treat myself to some discounted Fox 34s and a Boost conversion kit for the front wheel.