I used to think the same thing before I went to auto mechanic school. In the transmission and driveline class, the teacher (who had been teaching mechanics since the early '50s) asked us the same thing. The entire class said they had to be the same. He pointed ut that with most 4x4s, the front and rear axles have different ring gear sizes, so the ratios will be slightly different than the advertised spec. Most manufacturers will round it off to the nearest common ratio. So inthe case of the truck that Craig got his axle from, the rear axle was most likely a 4.10.
