I have a 2001 Seville STS and I am having a similar problem. however, my front end doesn't dhake, my seats in the passenger cabin vibrtate. This happens on smooth pavement at betweeen 65 and 80 mph.
I saw there was a technical bulletin abou this Here is the link if you are interested.
I am at a loss as what ot do . I had new tires (michelin)put on, balanced by tiure dealer, balanced by cadillac, yet I still get vibration.
The link to the article :
http://www.alldatadiy.com/alldata/AF...63452/94365049
Mark Elias