I say a toe in check would be in order. If the pads hit square on the rim or toed out even a little, they can pull forward, release, then pull forward again. This would cause the shudder. With beefier brakes such as newer dual pivots, squealing might be silenced, while vibration or shudder may not be. Of course the spacer could have made things a little worse, but I think the main culprit would be the angle at which the shoes contact the rim. Put a medium size zip tie flat between the brake shoe and rim at the rear, squeeze the brake hard, loosen the shoe, then retighten. It may help, or may not, but it's worth a try.,,,,BD