In off road circles this is often referred to as "Death Wobble" and I think for good reason. I just about soiled myself when it first happened to me. Searching google will come up with many interesting "solutions" to the problem.
This is one of my favorite threads on the topic...
It doesn't sound to me like you have 'got it' yet. If I am wrong than I apologize.
For me the easiest way to wrap my brain around a suspension like yours is to think of it as a 3-link. In essence the TA is just a 'very long strange shaped link bracket' just like the short brackets that...