It sounds like the front pinion bearing is missing, or it's lost all its bearing rollers. Take the pinion nut off, remove the yoke and front seal, and see if there's a bearing behind the seal. I'll bet you find no bearing at all, or only the inner and outer bearing races, no rollers!
Will the seller supply you another rear axle? I know used parts are sold "as is", but most wrecking yards will do a swap rather than a refund. (assuming you got it at a dismantler)
Jerry
