Trailblazer - You still got problems?
I just found out about someone who had a shimmy at highway speeds. Turns out that one of the rear brake drums lost a big chunk of rust and put that assembly out of balance. They tried tires, balance, driveshafts checked and new u-joints. The garage mechanic couldn't find it. What they did was run the truck while on a hoist so they could view the driveshafts, etc. and then the tried 4x4 & 2x4 and then tried with one wheel off at a time and that is how the found it.
-- Regards Gordie