Have you checked to be sure your front wheel hubs and bearings are okay?

 

I have a 2011 quad, 2wd and 4.7. It has the same rear brake shake that frustrated​ me on the 2005 I had.

I changed the front rotors with Centric slotted and powwrstop pads. Also installed new rear drums, shoes and hardware. Problem is 95% gone but you'll still feel a mild shimmy in the rear after driving a while. Tolerable but not ideal.

 

On the 2WD quads, I believe there's a 2 piece driveshaft with a center bearing that wears and sends the driveshaft into harmonic resonance sometimes.