I'm new as far as posting. I've followed this Forum for quite a while and found it both helpful and interesting. I have a 2000 Bonneville with 204K that I handed down to my daughter. It has a bad vibration under acceleration only. Smooth as new while coasting. Vibration starts @ 45 plus mph. I replaced the tires, outer tie rods, right lower control arm, and left front transmission mount. No change what so ever. Front wheel movement is very slight and about equal when checking for side to side or up and down play. Right drive axle has about twice the end to end movement as the left. Should there be any movement end to end? I read through all the previous threads and didn't see anyone with the same problem, unless I missed it. Can anyone shed some light on this for me?

Does this vibration get worse when braking?

No. It doesn't vibrate while coasting, braking, or even turning. Even at high rates of speed it'* fine unless you are giving it more power. The vibration gets worse with speed, but only under power. Goes away completely as soon as you let off. I'm not a professional mechanic, but there isn't much that I haven't done. However, this one really has me stumped.

You will want to check yoru axels for play. Billboost has a write up on checking the suspension. I'll see if he can get that finished for posting.

It'* finished for what is needed here Dan. But that write up only covers balljoints, tie rods and wheel bearings.

If the issue is only accelerating, have you checked the front motor mount? The 2000+ has a common failure of the front mount.

When you say the axle has play..how do you mean that, it should be held solidly to the wheel hub and connected firmly into the trans leaving only a little play in the center of the shaft.

I suspect his axle. The one that he said has the play.

Thank you Dan and Bill for your replies. I replaced the hydraulic tranny mount near the radiator. I do believe the engine still has too much movement when torqued. Which mount were you referencing? The one by the pulleys? As far as the axle play; I can move the shaft on the right side about 1/4 inch toward the tranny housing and back toward the bearing assembly. The left shaft only moves about an 1/8 inch. However, there is no movement up and down or front to back on either shaft. This has been the best car I have ever owned. I have only had to replace the crank sensor, oil sender, and cooling fan motor in 204K miles other than routine maintenance. I am planning to work on it again this weekend. I appreciate any advice you have to give.

Gentlemen,
It was the drive axles. The tri-pots had some slight wear, but the bearings were all ok. I can't believe that little bit of wear resulted in such a noticable vibration. Replaced left and right with new not remans for $125. Just wanted to follow up for anyone who might have this issue in the future. Thanks for your help!

Not a problem at all. If you need anything else, just ask.

Hey guys, I'm new to the forum. I just purchased an '01 SSEI the other day. This is the 2nd one that I've owned, but the I didn't own the 1st one for very long, so I'm still a rookie owner. Anyways, I'm having the exact same problem. I noticed the car vibrates a good amount at highway speed, but subsides when I let off the gas. Also, I can feel some vibration through the brake pedal when I'm waiting at a light. I was thinking it might be a motor mount, but could it be what Alford was going through? I'd appreciate any feedback about what to check.