[V8] Exhaust re-installed, but now we have vibration at idle

[Mark Kalbskopf]

This local shop just welded a new center section onto my very loud  
exhaust.
They had to remove the exhaust to do it, and muttered something about  
removing engine mounts to get the exhaust off.
I just picked the car up, and now I have this vibration of the whole  
car at idle that I'm sure wasn't there before.
I haven't had a chance to look underneath, but what mounts were they  
referring to? and how could they mess up the re-attachment?
There's no clanging sound or anything, just a pronounced higher  
frequency vibration throughout the car.
When I accelerate, it diminishes.

Mark.