Several guys I work with have 99-01 STS's that all have (had) 60-70 mph vibrations problems. The guy with the 01 has had the dealer work hard to resolve the problem with rotors, several sets of tires, and even control arm replacements. Most of the focus has been on the front end. The dealer even did the force balance and tire sorting thing with no improvement. I do most of the work myself on my '99 and I have been able to significantly improve the issue with ... it sounds wierd ... REAR rotor tuning. I call it tuning because the rotors are within spec, but making them closer to perfect seems to make the only difference. I also had to loosen the parking brake cable on mine which helped. The guy with the '00 just bought his so don't know how bad his is.
Bottom line: all three seem to have the same issue and I don't think it's tires. I think it's possibly the CVRSS system responce tuning up with brake rotor issues at the 60-70 mph speed and 'allowing' the bounce or resonation.