Hub to Drum Gap

To make a long story short, the apparently good enough solution is to add a spring clamp kit to the front and back of each stack of all 4 spring leaves. The "Superior 13-1401A Leaf Spring Clamp Kit" manufacturer part number is 13-1401A and the Amazon ASIN is B001DKSM7Y. Any residual vibration is virtually indistinguishable from even smooth road irregularities.

The problem vibration frequency was too rapid to be originating with the brakes, but before putting the rear wheels on I now always make sure the e-brake is set and the drum still looks very centered. Actually, the time the drums looked most off center was when the gaskets were still keeping them stuck tight from the factory.

Toyota replaced the springs for this problem on some of the 4WD trucks, so I don't know why they aren't doing anything for the 2WD trucks yet.

 

For a softer ride, just using the clamps in front of the axle is an adequate compromise, but in any case, the long term warranty effects with this modification are unknown.