Intermittant vibration

[mark.e.woodland at gsk.com]

After replacing the stock T44 suspension with Bilsteins/H&R progressives
and swapping the new tires to the front, the car (Max, 87 5KTQ) picked up
an intermittant vibration.
At 60-75 mph, the car will be smooth for 10-15 sec, then vibrate for 10-15
sec.
Could the difference in diameter between the new (fronts) vs. old (rear) be
torturing the center diff?
I'll be getting new rears installed soon, but could I have adversly
affected the trans/diff assy?
Any BTDTs or horror stories?

Best regards,
Mark Woodland
Max '87 5KTQ 265K
Gina '86 5KTQA 240K