Yesterday I changed all my wheels to a set of subaru alloy rims from a subie I am decommissioning. I have been driving that car for the last 6 months and there were no issues with the wheels.  First time I go to put on my brakes there was a violent brake shimmy. They seemed to go on fine and I can't think what could of changed to make my brakes so weird. I'm going to try loosening the nuts and torqueing them to 80 ft lbs.

Re-torqueing hasn't helped and the shimmy is still there when braking , worse at higher speeds. Scary in fact. Today I tried to do an E brake stop at high speed which while slow stopping did not produce any shaking. This would lead me to think that the problem is in the rear drum brakes, so I lifted the rear wheels off the ground, and with a pole, put on a slight brake pressure with the motor running. The results were inclusive. While there was a small difference in drag when I rotated the wheel by hand, it was the same in both wheels. I'm trying to isolate the problem but not sure how to do it. Can't tell where it is because it makes the whole car shake. 

Might try backing the rear adjusters all the way off, one side at a time, and try driving.

I would be checking out the front lower ball joints...