today i noticed when im driving less than 20mph i can feel the car wobble a little in the rear end. it feels like riding on small bumps or something. but its like a continous type thing, where it wobbles every 1 second interval. it only does this if im going less than ~20. other than that it cant be noticed at higher speeds. could this be due to messed up alignment? the tire pressure on each tire is the same. im just clueless as to what it is. any help would be much appreciated. thanks.