No it is not the tires or rims, replaced both axles and hubs, changed
top motor supports and lower motor motor, replaced tranny mount, even
changed spark plugs, wires and coil packs. The car starts to vibrate at
35 mph especially going up a hill, then mellows out after around 45 but
you can still feel the vibration but not as bad as at 35 mph. I have
had it at a GM dealer and they are scatching their head.  The car has
110,000 miles, now they are telling me it might be the planetary gear
in the tranny, the tranny was rebuilt at 77,000.  Has anyone out there
had any problem with their Grand Prix?