My 2004 Grand Prix vibrates between 50-70 miles an hour when you accelerate or under load. If you let off the gas the vibration in the steering wheel stops. I have had it scoped, tuned up, injectors flushed, but still the same.

a problem axle will vibrate worse the faster you go but the vibration will be there up or down so for the moment I will set that aside.
if it is vibrating during acceleration but stops when the accelerator is released, it indicates and engine /transmission management problem
first I would run the fault codes as the problem could be from a faulty cam position sensor
servo/solenoid that has the cam too far advanced for the engine rpms and power setting
next it could be a problem with injector control or fuel flow/ pressure
have the fuel pressure tested at rpms to ensure that the pressure regulator is operational
what you describe occurs from misfiring or the like so go to an accredited service center for a proper diagnosis and quote
back to the axle --it could be a problem wheel bearing failing or needing adjustment so keep that in the back of your mind if nothing shows up in the fault codes or diagnosis from a shop with experience

Hi Cheyenne:
- For the vibration, I'd be checking universal joints or CV jounts.
- Sounds like you've got your brakes figured out already. Time for some maintenance.

if the wheel alignment is faulty ( castor wobble ) at 70 mph there is no way that you can describe it as a vibration as it virtually rip the steering wheel out of your hands. If it is a torque converter problem then at around 3850-4025 rpm stationary there will be a vibration of the vehicle and engine The sort of vibration that you are experiencing at 70 mph will come from a drive line problem. If rear wheel drive then it could be faulty "U" joints, worn slip joint or failed rear engine mount
If front wheel drive it may be cv joints or bent drive shafts,wheel bearings or brake callipers not moving freely on the mounting bolts.

were the motor & trans mounts checked?

bad engine mounts
bad ground point
low fuel pressure causing random misfires

Did you look under the hood and try to pinpoint the noisy component?

try getting motor mounts checked or struts or wheel bearings , start with mounts.

wheel bearing

check the idler arm on the steering drag link

check wheel bearing