Vibration on Audi A4 Avant Quattro 1.8T starting at 50MPH?

I have recently acquired a second hand Audi A4 Quattro Avant 1.8T with AVJ engine. All is fine with the vehicle except with one noticeable problem; a vibration that starts at about 50MPH going all the up to 80MPH on the front passenger wheel. I did not try going faster than that as I think it might be unsafe to do so. The problem occurs ONLY when accelerating. i.e. driving at 50MPH downhill does not cause any vibration what so ever, however driving 50MPH uphill causes the vibration. The vibration gets more violent with increasing speed. The vibration is considerably more noticeable when on FIFTH GEAR; is present on third gear but negligible. My mechanic have replaced the Track Rod End, inspected the wheel bearing and Bushes; all is fine. We have replaced wheels, done wheel balancing. Nothing helped. What could it be? Any help and suggestion will be much appreciated.