Try 'stab - braking' where you lean on the pedal and then let off..... if shimmy goes away in between stabs - then its gotta be the brakes.

If its warped rotor you should also feel it in the pedal.

Also try noticing if car shakes only at specific speed, say 35 or 45 mph...... shimmies at specific speeds are usually related to front wheel / tire ballancing.