when I get it up to around 40mph, the deville starts pulling to the right and I have to adjust and then it pulls, etc...it's like driving on ice. any ideas what would cause this behaviour?

Seriously worn suspension/steering/brake components or alignment. A good alignment shop will give you all the grisly details.........

thanks for the advice submariner409. I've been researching this further and I believe that it might be related to the rear knuckle bushings. The rear wheels are a little sloppy from what appears to be worn knuckle bushing and I believe this is causing the car to kind of float/pull etc..

I'll update once I have it nailed down for certain

So my first thought that the car was "pulling" was a bit off. It was more like the rear end was really sloppy causing us to compensate constantly while driving. Very unsafe.

I pulled the knuckles, my local auto shop pressed out the old bushings and pressed in the new ones ($69.22 per knuckle on EvilBAY) and I no longer have that unsafe rear end floating. I'm very happy.