My first thought was a wheel bearing. I've had one go out on another car before and it did exactly what you are describing.
I wouldn't rule out anything based on milage. Trust me when I say that things fail sooner than expected more often than you would think.

 

A couple of things you can try, to see if it is a wheel bearing, lift the tire off the ground and try to get it to shake, if the bearing is bad and making vibs and noise, you should feel something like a loose feeling and it should rock back and forth. Normally they are very tight. You might also want to look for cupping in the tires, the rears are usually where this problem starts. If the tires have not been rotated, they may be cupped and cause the annoyance. It happened on a friends mazda suv, the back of the vehicle is not heavy enough to wear the tires properly, and bouncing occured, and uneven wear. I wouldn't suspect this on an srt4 but, check it anyways. If all esle fails, spend the 50 bucks and have a tire/front end allignment place look at it. A reputable(sic) shop should be able to locate the vibes, thats what they specialize in. And it should be relatively cheap, your only paying for the inspection. Which here at tires plus is like 20 dollars. And yes they do caught things, they informed me when I had a bad wheel bearing on my golf.

 

Unfortunately the SRT-4's are often times abused and can easily have worn parts with little mileage on it. They are built to perform and have plenty of road noise from the tires even when new. A front end shop should be able to find any problems, but make them show you the problem and any parts that need replacing before you have them do any replacements.

 

Yeah, don't rule out the wheel bearings, I just had both fronts replaced at 6500 miles, and I ain't been that hard on this car. Hope you get it sorted soon.

 

my factory tires wore unevenly and they sounded like mud tires. i even dynamated the whole car and it was still loud. got a new pair of tires and it was all fixed.