Do you have any vibrations in the steering wheel when you are applying the brakes? As if the rotors are uneven or warped?
I had the exact problem you describe and the problem turned out to be in the front rotors. I had some pulsing when braking but I never thought that a rotor problem would cause vibration when the brakes were not being used. It was not until I turned the rotors that the vibration while driving went away. I cant explain why or how the warped rotors is causes the chassis vibration, but it was.
If you dont have any brake pulsing, then I am clueless.
Let me know what you find if you ever get it cured.
I got too many cars!! Insurance eats me alive. Dave
78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k