Some dealerships do and some don't. It'* like any mechanic....some have a higher sense of pride.

It is my experience that the brakes on our cars are rather sensitive to warped rotors. I personally machine my rotors myself on my own lathe (I have a machine shop) because the accuracy that seems to be required is not something the average mechanic with an average machine is going to provide. The "Specs" for "acceptable" tolerances are loose enough that your rotors are probably within those tolerances now. If you can get the rotors turned 'on the car', that will probably be the best, but I don't know if the rears can be done that way.

At any rate, you'll want to take your car to someone that is very good at what they do. Some cars are not as critical, but Bonnevilles seem to be.

Ive been chasing down the root cause of the shimmy too. Yes it occurs only at the noted speed and beleive me I have tested many speeds... Curiously enough it shows up at slightly different speeds depending on whether I was driving into (shimmied only lower speed) or with the wind (shimmied only at higher speed).

Although I have not had the road force balancing done I have had many balancings and rotations and replaced rotors with no effect. The only thing that appears to have reduced it to some degree is I had a rear left endlink adjustment arm replaced because a mechanic noticed it was a little worn/loose and this appeared to reduce the shimmy by about 50%. I am now wondering whether I should get the other one done too.

Alignment is critical. It needs to be checked by a real professional who will set it to exactly the middle of the acceptable ranges, not just say, "It'swithin toleraces."

You also need force balancing and you need quality, round-rolling tires. You also could have a rim slightly off but that will show on a road force balancer.

My final adjustment came after I told the service manager at the dealership that on a newly-paved I-65 it seemed to do it more pulling slightly uphill on a long grade and didn't show it on the way down. I figured it was front wheel toe-in varying with the pull on the wheels. Rear toe-in was off slightly. The car has Michelin Symmetry tires on it. With miles up to about 15K they seemed to even out and soften in their rolling so the variation was less and less.

Its simple, drive normal untill you hit 60mph then floor it till you hit 70.
Sorry but I had to say it. welcome to the club.

just about every time i get a speed(50-70)sensitive shimmy/shake its a dam tie-rod end.
3 times now and each time its one of the tie-rod ends.

ROD

<<I just wanna have a smooth ride 24/7>>

There are MANY of us who have experienced this problem, and still do. I feel certain you may have to live with this. I know there are those of us here who will disagree and some will agree. Tires and balancing, and rebalancing, ad infinitum, may not help either. The 2000+ Bonne, especially the SSEi, is known for this anomaly.

It may be a symptom of "cumulative" issues: imperfect tires (non are perfect), light-weight aluminum suspension, light-weight aluminum wheels, imperfect wheels, imperfect tire balance, stiff sport suspensio and on and on.

I'm sorry, I'm afraid I did not notice whether or not you even have the SSEi but Bonne'* and other full-size GM'* are known for this tool. I don't know how to go back and look to see what model you have without destroying this post.......... so I'll post this anyway.