I have a vibration problem when accelerating between 40-50 mph. I drive a 2003 Buick LeSabre. It gets worse when I have some passengers or going uphill.
I recently changed brake rotors, brake pads, and outer tie rod ends for both sides; the tires are 1 YO and it was regularly aligned and balanced.
But the vibration remains the same.
what would be the problem?
Hard to say exactly without looking at it of course. But I do have a few ideas that might be worth trying. In my opinion I am thinking it needs a tune up or a coil pack is failing. My reason for thinking this would be because you stated it gets worse under load. This is common for an Engine Misfire. Pull any engine trouble codes that are stored.
If you feel the vibration in the steering wheel or in your butt you can perform a quick test here. Swap the two front tires with the two rear tires leaving them on the same side. Then test drive it and see if the location of the vibration has changed. If so, you know the issue is tire/wheel/balance related.
Sometimes but not all the time does the check engine light come on when a misfire occurs. But if the check engine light is on the stored code may be able to indicate the exact cylinder causing the problem. Engine vibrations can be caused by worn spark plugs, plug wires, ignition coils, etc.