I am certainly no mechanical engineer, but it has been my experience over the years,and with several different bikes, that the high-speed wobble we all affectionately refer to as "tank slap" is directly proportional to the overall condition, and more specifically, the BALANCE of the rear tire.
Check it out sometime. Ever notice how a brand new back tire changes the characteristic of the bikes' need to tank slap? I noticed this with my old XS1100 some years back and even had more than one rear tire re-balanced with a spin balancing machine - with great results.
It has always seemed to me that any issues with a FRONT tire caused more low speed problems such as a slight shaking of the bars, but any issues with the BACK tire caused the tank slap.
Has anyone else noticed this as well? Or am I nuts?