Originally Posted by dgi 07

check the rear rotor flex. its a rubber coupling between the rear driveshaft and the differential.

Also, this my seem like im crazy, but Check your tires as well. Check to see if you have thrown a wheel weight. Also check them for water ingress. Remove the valve stems and see if you get water spray coming out. I know it sounds wierd, but my last set of tires had water in them and caused a similar vibration. Also, there should also be a grease plug on the rear u joint and slip shaft. Lube those up.

I am assuming you have modified or replaced your front driveshaft?