Vibration

[TFENT]

So I have a 90 ZR1 with 31,000 on the odometer, totally stock except for a Marc H. chip. Also live in San Diego where we can open up the cars more then most people (ie 100+) on the freeways. So I took out the vette for a "Sunday" cruise this weekend (150 miles). The car has always had the Corvette creaks and rattles but anytime I am over 60mph there is a vibration that is irritating my beyond belief. The car has new tires and rims and I have had them checked like 4-5 times on a load match machine. They are perfect. Thing is this is my first Vette. I do not know if these cars have a natural tendency to have a high speed buzz or not. Since It was sitting a very long time in storage before I got it I am wondering if maybe the drives shaft(s) or U-Joints are bad/frozen or out of balance.

The buzz is just enough to cause the rearview mirror to vibrate enough to not see out of it. Its not real bad like a flat spot. Just as i stated earlier it's an irritating buzz. especially at 110 when I NEED the rearview mirror.

TIA for any suggestions.