Wife's 2005 Toyota Sienna 3.3 AWD vibrates badly on the highway and is loud. I thought an out of balance wheel so I rotated them to see if the vibration moved to another location but there was no difference. Figured it must be a wheel bearing so we took it to the shop. This is where things seem weird. They say that in the section of double walled pipe between the headers and rear cat (flex pipe and y pipe) the inner pipe has disconnected and is rattling around causing the vibration and noise. I can hear that it is lose and rattles in the pipe, but I don't feel an exhaust leak. Can this really cause that bad of a vibration? The whole car shakes and it is loud enough that it can be difficult to have a conversation. The vibration is worse around 30 and 70 when holding a steady speed. It almost stops when letting off the gas. I was going to have the shop repair the exhaust but each time I talked to them they quoted me 3 different prices from $300 to $550. I can buy both pipes online for $130 shipped vs $250 that they want for them, so I will be doing this myself and buying replacement o2 sensors just in case I can't get the old ones out. Thoughts?