I have had my 99 for many yrs now and I really haven't been on the highway that often for the last few years but when I go faster than 70, I get a slight vibration from the dash, below that it is smooth. I have balanced my tires and rechecked and they are fine. It also does it on my new snow tires and wheels. The shocks are good and the suspension is tight. It is making me crazy and no I will not drive below 70

 

I again checked the rear and I don't see anything wrong, I was hoping that someone had an idea.

 

start by rotating tires front/rear to see if something changes. After that, I'd jack up the rear wheels and rotate by hand. Any brake drag or a grinding feeling? I'd look at brake pad/rotor/parking brake shoes as a possible cause or a wheel bearing. Is your car FWD or AWD? if its AWD, then I'd check the drive shaft bearing.

 

All great ideas above. Other things to check when you have the wheels off. Check for deposits on the inside of the wheel, mud sometimes gets stuck to the rim and can cause this issue. Look for flat spots on the rims, inside and out, these can fool a balancer. Roll the tire when you have it off, look and feel for any deviation in the tread pattern. Lastly, check to see how it was balanced, how much weight was used and in what places.

 

I about to put on all new rear rotors and pads. I have rotated and put on brand new conti snows and it still did it. I hope the the new rotors would fix it.