I was looking in the "Catalog of Chevy truck ID numbers 46-72" for rear ends that would match mine (57 1/2 ton.) In 59 the ratios & stamped codes on the 3100/3200 series were as follows; 3.9:1 AF or AR, 3.7:1 AJ, 4.11:1 AV, 3.38:1 AY, & 3.92:1 (no code listed). Numbers after the letters indicate date made. Example: AY 212 is 3.38:1 built Feb 12. The codes are stanped on the right front of differential carrier flange.
As for the $400 for a new ring & pinion (3.38:1) there are some places that sell them, plus another $210 for install kit and another $65 for speed adaptor plus shipping and handling. Golden State Parts is one company that sells the set. I hope this helps.
