New wheels, new problem PLEASE ADVISE

[Y2KVert]

Got my new I forged wheels mounted a couple of weeks ago. We had to use spacers in the rear to clear the calipers (I am running Baer +2 rotors) the spacers are around 3/16 and we still had to grind on the caliper to make them fit. When the wheels were ordered I told them that i was running big brakes anyway they didn't fit, hence the spacers. Here is my question, around 70-75 mph I am getting a rythmic vibration. At first I thought it was from the front wheels as the rings for the hubs didn't seem to fit right, so I took them off and reinstalled the wheel. The front seems to be fine now but I still have the vibration in the rear now. Maybe the spacers are causing the problem? Could this be a tire balancing issue? I feel no vibrations at all until around 70-75mph and as said before it is a rythmic vibration. The spacers we used were from a local parts house and seem to be generic. Maybe new spacers? I'de like a little more info before I go grinding on the calipers again.