[davemon]

Update: I got the 1 known culprit tire balanced, 2 adhesive weights laid inside the wheel. Pushed it to 70mph and still some chatter but not from back there and the car felt better, not by too much though.

I returned, got the other 3 wheels balanced. Pushed car to 70mph felt great. Took the car back out tonight (after traffic was gone) and pushed the car to ...ahem... very fast... and no chatter! no vibrations whatsoever. For some reason these wheels weren't balanced well at all. The shop owner said BF Goodrichs can be hit or miss and sometimes they wear weird and fall out of balance. So whatever that is worth. He did fix the previous shop's dumb mistake though and got the car perfect.

All this for a measly $10/wheel. Wow. +1 for Austin Tire Man on Manchaca.