The cheapest thing to try first is to balance all 4 wheels.
Thats what I did and i found that I had a bad rim on the front.
I hit on of many thousand of potholes this spring (guess I couldnt avoid em all)
Anyways what I ended up doing was putting the bad rim on the rear which almost eliminated the shudder. I get one now at 140km/hr which is so slight its hardly noticable. I think its because I look for anything wrong and know the rim is bad. I was getting Exactly what you described.