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Could be several things, I guess I would first ask what order were things removed and replaced- if you did the job? There should be very little end play in the axle but with a stock posi and no other work other then the axle being replaced you may find 030+ of play. If you really hear a hard knock I would not drive it until you can figure out where it is and what is going on. If you checked it while up in the air as Gusto said then it could just be a binding u-joint. if you heard it driving stop and figure it out.