Take your tire in question to a local tire shop and have the balance checked.
While it's on the balancer (spin type) have the tire teck check for belt separation, out of round, uneven wear, or bent rim. You might have just lost one or two of those stick on wheel weights. Another thing, water might have got in the tire from an air tank that wasn't drained daily like it should be. I've had tires with gobs of water in them. I always check the air hose with my finger nail just to make sure it's not full of water before i air up just to be sure. Bad places are convenience stores with slackers who don't do their job and drain the tanks daily.