The numbers on the rear should be stamped along the flange where the case halves go together, post those numbers and we'll decipher them.
The tranny has it stamped as well, not exactly sure just where....
The Muncie assembly code ("P"-number) is stamped on the vertical rib at the rear of the main case on the passenger side, and the VIN derivative is stamped in the same place on the driver's side.
The casting number for the diff housing is on the passenger side of the nose, just above the horizontal "fin" on the casting, and the casting date is on the driver's side, partially hidden behind the front edge of the camber strut bracket. The ratio code and assembly date are stamped on the bottom flange of the housing inbetween the spring retaining plate and the camber strut bracket. Post what you find and we can decode it for you.