[Jac Mac]

Since you have a balance bar find a real quiet or private stretch of road, dis-able or remove the pushrod to the front brake master cyl and try it with rear brakes only, then reconnect front brakes and disconnect rears & try again. That should help decide whether its caused by the brake rotors/calipers & at which end of the car.

Also remember that jag rears with the fixed length halfshaft & u-joints can suffer from the old unequal u-joint angle problem, is there enough deflection in the rear end geometry to induce this condition under braking...eg toe in or out in combination with ride height. This could be the case since you get no pulsation in the pedal. What happens is the half shaft accelerates/decelerates twice in each revolution at one end in relation to the other, since you have your foot on the brake the tire end does the acel/decel bit and you feel it through the car or to be more correct-your butt.