I have an 06 Jeep Grand Cherokee 4x4. I live on and travel several miles of dirt road everyday. In the last couple of weeks I have had an issue when traveling over washboard roads. I will be going along fine and then all of a sudden when I hit the washboard bumps I get a very loud noise/vibration. The noise appears to be coming from behind the driver and passengers seats. If I put my hand on the floor in the back where it humps in the middle, I can feel the vibration (almost like something is hitting the floor there). If I put the car in reverse and back up a little and then drive forward all is good again. I have been to the shop and the suggestion is that there is a bad u joint. I am told we can't replace just the u joint, that we have to replace the entire shaft for $900 (Part only). My concern is that I have no issue while driving on paved road. If my U joint is bad, wouldn't there be other signs as well? Any help/advice would be appreciated.

You can do the ujoints without replacing the shaft, a drive shaft shop can do it for you and balance the shaft too. But if it was a bad ujoint, you should have noise and vibration even on smooth roads too, I would think.

That's what I was thinking. Any other ideas as to what can be causing this problem. Has anyone else experienced a similar problem?

Guess it depends on how bad the ujoints are. Those wash roads can be pretty rough, might be indicating something that isn't present on the asphalt yet. Wouldn't rule them out.

Is there anything else I should check before assuming it is the u joint?

I should have added - This problem first started a year ago in the spring when the back roads are rough. It happened a few times in rough areas and then as the roads improved I didn’t have any issues, until now, 1 year (almost 20,000 miles later) when the roads are in rough shape. I don’t have any issues on the pavement, this only happens on rough washboard type roads. I have even hit larger potholes and it will not set off this issue, only when the road is uneven so to speak.

If something is physically contacting the floor pan, you should be able to see it.

Go make it do it again and before reversing to correct it, stop the truck and get under it to see if you can see what is making contact.

The other posters are correct that a bad ujoint would not be intermittant.

I will try that. Thanks for the suggestion.

hows the exhaust hangers

I will have the exhaust hangers checked. Thanks for the suggestion. I would love to get to the bottom of this.

Was this problem ever figured out? I am having the same problem. Your description fits my problem to the T! And my mechanic is confused. I would love to know what it is...