Jack up the front on one side so that wheel is off the ground and make sure it's solidly on a good jackstand. Set the parking brake and chock the other front wheel. Start the car and let the wheel that's off the ground spin up to speed and check for vibration/noise. If you can't find anything, jack up the other side. This only works of you don't have LSD.
I had an outer joint (NAPA rebuild) that developed a tight spot that was actually causing the engine/trans to shift side to side. By running the car this way and the hood up, I could actually see the movement and I knew exactly which axle it was. The "tight" spot was almost impossible to feel with the axle off and I had to argue a bit to get another one since they didn't think there was anything wrong with it.