I know it's going to sound "crazy"....but double check your firing order, make sure the plug wires are on the right cylinders and cap terminals....I bought a truck once that a shop sold cheap because all the ASE certified techs couldn't solve the drivetrain/body vibration it had at low speed.... took a few minutes to find it after ruling out the simple stuff...never know?
I know it's going to sound "crazy"....but double check your firing order, make sure the plug wires are on the right cylinders and cap terminals....I bought a truck once that a shop sold cheap because all the ASE certified techs couldn't solve the drivetrain/body vibration it had at low speed.... took a few minutes to find it after ruling out the simple stuff...never know?
Worth a shot
Cheap is good
A vacuum gauge may show something as well
the puzzling part is it was fine for the first 200 miles and then occurred after that in which time I assume the car was parked for a period ? It has to be engine/trans related based on the details.
Only other thing I can think of right now would be to check exhaust temps before and after the mufflers. might have dropped a baffle or have a nest but those are long shots. Cheap to check though
keep us posted
