on my 850 I had the same thing.. replaced all rotors and I still had abit of vibration. I checked for anything loose and it was still.. I replace one of my stabilizer end link and it was gone.
I determined it was the problem by grabbing the endlink and shaking it. it would give a small snap. It looked fine but I replaced it and problem solved.

I have a 97 850 T-5 and I have the same or similar vibration. It felt and sounded like the vibration was coming from the rear. I haven't changed the rear rotors but did do the fronts. It seems to be getting worse and now, its a full shimmy at 75 mph. I didn't get a good feeling about the reply you got. Did you come up with a solution? shoot me an email at alaphil@hotmail.com if you do. Thanks.

D.

i would check if u have any loose suspension parts in the front that occurs when ur front end compresses...


this may sound stupid...but just checking... c if u have all the guide pins for the rotors installed and properly torqued..


also, make sure when u put on the rims, u should put the torque of the lug nuts to 81 lb/ft... it doesn't matter if u get new rotors... if u continue to over torque it... it will warp.


another thing to do is to properly seat the pads and rotors. look at the rotor runout... as well as the hub runout. properly seating the rotor and pad allows the production chemicals on the surface of the rotor and the pad to burn off and not build up between them....