Vibration in Gas Pedal

When I was driving home tonight I noticed that there was a slight vibration in the gas pedal. I checked to see if the 4 wheel drive was on and it was switched to A4WD, I don't know how and when it was switched. The road was wet but not bad enough to have it in A4WD. Is this normal to have a vibration in the gas pedal? I changed it back to 2wd and the problem was gone. Never had this problem before in A4WD.