Help with tire vibration

I just purchased my H3 in Jan 07 and after about 2000 miles noticed a vibration from the what I believe is the rear passenger tire. I have the stock 265's. I just took it to the dealer and they told me all 4 tires were out of round and replaced all 4 and balanced. It does not seem like this has corrected the problem. I really notice it between 50-60 MPH and it is not constant, every 3-5 sec. It feels kind of like a tire with low pressure if that makes any sense. Can anyone help??????

I'd take it back to the dealer!

I find it hard to believe that all four tires had issues. One, yeah ok, but 4. I think it has to be something else. Maybe you have a bad rear brake thats dragging, or a warped rear rotor? If all four tires/wheels were bad, you would have known it, but wouldn't have been able to tell it was comming from one area.
 
I'd make 'em look it over again.

Slightly bent wheel..??
 
Bent axle..??
 
Something wrong with driveshaft, bent, out of balance or a midget hanging on it..

Sounds like something is bent.  I also can't believe that all 4 tires are to blame.

I would agree with Crash. Something is bent or out of balance in the drivetrain somewhere. It could be a bad shock too. Look under the rear of the vehicle at the shocks and see if one or both are leaking any fluid.