Changed the oil last night, and was going to rotate my tires to see if the vibration moved and showed up in my steering wheel so I could verify it was a tire, and not suspension/bearing. Got the front jacked up and on stands, went to put a 2x4 behind the rear wheels, and noticed the left rear was almost sitting on the rim..."Well, there's your problem right there..." Finished the oil change, aired up the tire, took it for a ride and sure enough, the thump was almost gone...still noticable, but not what it was. "Yup - glad I got that bonus in my check on Friday so I can get 2 new tires next week." Got home and thought a giant snake was attacking my garage...but no - just the tire losing air faster than Justin Bieber loses respect. So, threw the spare on, and got 2 new shoes put on this morning. It's very happy now...