2001 GTP vibes/shakes while accelerating

I've got a 2001 Pontiac Grand Prix GTP with 151k miles. For the past two months, I've had some horrible shakes and vibes while accelerating. The shakes and vibes are proportionate to the rate of acceleration. On the interstate @ 75mph, the dash, steering wheel and A-pillars are noticeably shaking. It's felt through the floor and seats, too. Shakes started in January just after having new tires put on. After the new tires put on at the local Sears store, it drove PERFECTLY for the 20 miles back to the house. The next day, it drove perfectly again, for 20 miles to Firestone to get the lifetime alignment checked. IMMEDIATELY upon leaving Firestone, I could feel a very slight vibe in the car. It was so slight, that anyone unfamiliar with the car would not have noticed. Over the course of the next couple days, it increasingly got worse, to the point of the shakes. Took it back to Firestone, and they told me I needed cam bolts in the struts to make the alignment proper. Changed them, no fix to the shakes. Took it back to Sears to have the tires checked, rebalanced three tires, no fix to the shakes. Back to Sears again, where they told me my ball joints were bad. Changed them, aligned at Firestone, no fix to the shakes. Firestone then said the trans mount and CV axles were bad. Changed all of them, STILL no fix to the vibrations. Every time I take to an "expert", they tell me something different. I fix/change that, and there's no fix to the car. Several friends suggested bad tires, but the shakes are only there when accelerating. The second I let off the gas, the shakes disappear, and the car is as smooth as can be. Any ideas? Anyone? I'm at my wit's end, and don't like the idea of having to take it to the dealer to pay them $100+ dollars and hour! Help, please! Thanks.

Welcome to AF!!

First of all take the car to someone other than Firestone and Sears to have it checked. It sounds like they are just trying to get your money. If the problem started after an alignment it may be something in the alignment that is causing the issue. If it only happens during acceleration however, it may be the motor mounts especially given the fact that you have had everything else changed. The only other thing I can think of would be control arm bushings may be bad. I was originally going to mention the CV joints but you have that covered already. Good Luck!

Thanks GTP Dad. I concur that Firebone and Sears are pretty much not the answer, and have come to realize that I'm going to most likely break down and pay a more experienced mechanic (I just hate paying for work I could normally do my self!). I've been going to Sears since I have the tire rotate/balance warranty through them, and Firebone because of the lifetime alignment I have there. I've also considered the control arms/bushings too... They are, after all, 11 years old and have 151,000 miles on them. They certainly don't owe me anything, anyway. Thanks again! GNPilot

Mine did the exact same thing and it turned out to be the CV axles. But like Dad said, you replaced those already. Is it possible your replacement axles were crap?

Thanks 00GTP4ME,

As it turned out, I wrote my previous follow up post after changing the left axle (with no fix), but before changing the right axle--which did actually fix the problem. Talk about playing stump the mechanic with this one... no one could and/or would believe me that the CV axle(s) were causing this problem.

Thanks again!
GNPilot