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Wobble at low speeds turns into thumping at higher speeds

2000 Pontiac Grand Prix. The car has a wobble at low speeds, 20-35 mph, that turns into a thumping at higher speeds. It does this when accelerating and coasting. It seems to be in the rear of the car and not the front. The whole car wobbles from side to side. I do not feel it in the steering wheel. I am thinking that it is something wrong with the tire, but not sure. I did find that the rear tires were over inflated, one was 15 lbs. high. Heavier loads in the car make the problem worse. Any Ideas?

First, check the lug nuts on all of the wheels, as it could be something as simple as a loose wheel.

However, if all of the lugnuts are tight, DO NOT drive this car, as it is potentially very dangerous in this condition!

Have it towed or flat-bedded to a competent mechanic (NOT to a chain operation or a tire store) for examination.

Listen to VDC, this could be a crash-the-car kind of problem. Get it figured out ASAP.

Lug nuts are tight. I did just go and shake the tires back and forth. The driver side rear is sloppy, and it clanks when I shake it. Is this a bad wheel bearing?

Sounds like a broken belt in a rear tire…You can see this by simply watching the tires as you drive by slowly in a parking lot or follow in another car. You will see the deformation in the tire as it rolls. The solution will be a pair of new tires. (it’s never a good idea to buy one tire). This damaged tire could blow-out at any time, especially at high-speed.

It very well could be and if “kicking the tires” makes them wobble - take VDC’s advice and do not drive the car. It needs prof attn before you get in it again.

Don’t think it’s a broken belt. OP said ‘it clanks when I shake it.’ Not a tire.

The OP never uses the word “clanks”…

He did say “clanks” in his follow-up post. Sounds like a rear bearing about to cause the loss of a wheel. The loose bearing is causing that wheel to try to steer the car, thus the wobble. Get it looked at ASAP. Towing is not a bad idea.

I thot he said it “clanks”

Thanks to all, I will have it looked at ASAP.

Apparently it is worn tires caused by a worn tie rod and bad alignment.

Thank you for returning to this thread to report on the outcome.

I assume that you are having all of the required work done.
Please do it!