Rob,
As usual Uncle Bob pulled us back to the right focus. I'm not saying that your frame is causing your death wobble, but there are some safety and stability issues there that I would recommend you address.

For a steering linkage that's not cross steer the first place I'd look for the death wobble solution is a loose panhard bar (which some call a track bar) mount, or improperly fitted/too soft bushings on that bar. Like Bob says though, it's the additive effect of tolerances that can cause the wobble, too, and it can be a bear to solve some times.

Best of luck tracking down the solutions to your problems.