I have that vibration problem . A mechanic told me that if the vibration occurs on pulling then it is in the drivetrain, I thought it was the tires. My vibration sounds and feels like when you drift over to the edge of the road and they have those warning ribs that vibrate the car as a warning that you are running off the road. I found the universal joints were bad and replaced them, I thought my problem was gone, but it is still vibrating. Mine occures around 65 - 70 mph, and if you let up on the speed, it stops, push down, it starts and if you just accelerate through the vibration , it stops. I can't figure where the vibration is coming from.

Check your pitman arm and idler arm at the front between the two front wheels.

Would I feel the vibration from the pitman arm, idle arm in the steering wheel? I don't feel anything in the steering wheel. Sounds and feels like its under the bottom. Anyway, how do you check the arms? Do you just check for motion or looseness? I did ask the Chevy service manager and he said the same thing about checking the pittman/ idler arms.

You would probably feel it a little in the wheel but mostly in your hips lol, if that makes sense. Just grab the pitman and idler and shake them, they shouldn't be loose at all. You should also check your balljoints. That would cause a shimmy when you hit a pothole but the shimmy would go away if you go off the gas.