If that spider gear isn't split in two, I'm going to guess a previous owner didn't get the shaft through it when the rear axle was last assembled. It likely wound up between the ring and pinion and caused the rest of the destruction.
BTW, your truck has 3.90:1 rear end ratio. See the 39-10 markings on the pinion. Also, that's the stock ratio for a 54 1/2 ton. I don't know if the 3.55:1 ratio gear set is available for the '54 models. The '54 half tons also have a different differential case than the earlier 1/2 ton trucks, so the available 3.55:1 R&P set may not fit. Look for the casting numbers on your differential case on the attached pic.
Kevin 1951 Chevy 3100 work truck Follow this saga in Project Journal Photos 1929 Ford pickup restored from the ground up. | 1929 Ford Special Coupe (First car) Busting rust since the mid-60's If you're smart enough to take it apart, you darn well better be smart enough to put it back together.
