I too have this problem at 50-65 mph. I also have 19" aftermarket rims wrapped in Nitto's. I have had two shops check at inner and outer tie rods, alignment and everything always checks out. I wouldn't say its severe but defiantly an annoyance.
Current: 2007 GTCS
*Roush> Supercharger, Suspension, Intake
*Exhaust> UPR OR, Borla Touring