Google GM Corp 2.5 ton axles and you will find many sites and write ups about them.
As stated 6.17 to 1 ratio.
Differential chuck and ring and pinion were made by Eaton as far as I can tell from my research. They are a HO110 ring and pinion. 12.5 or so inches in diameter.
There was three ratios made for this ring and pinion size. 7.20 to 1, the military version which we have is 6.17 to 1, and the holy grail if you find a set. 5.43 to 1. The 5.43 were said to be used in early 80's, C30 service trucks.