Just because tires were balanced a month ago does not mean a weigh didn't fall off. If you've been off roading and one has fallen off, the 55-60 mph range is the most common time to notice out of balance wheels. Everything else, ubolts, ball joints steering stabilizer, will be noticeable at more speeds than just at 55-60. Plus if you live in a neighborhood with really bad juvenile delinquents they may have played an old joke on you and taken them off! If this is the case wait for the zip tie around your drive shaft. You'll be looking for a drivetrain issue for weeks.