Axle Shaft Identification

[munztead]

I need some help, or I may be overthinking this but here goes.
I put a used rear end under my 78 C10 a few years back. Unfortunately, I can't remember what the junk yard took it off of other than a GM. I've gathered all the number I can off the housing and they are:

14055251
B276
GM34

It's a 10 bolt 8.5 differential, and what I really need to know is what year the thing was made. I believe it's a 83 or 86. I'm trying to order new axle shafts, but running into issues regarding the length. When I measure entire length, it comes out to be 31 7/16, which I can just say 31 1/2, but finding 31 1/2" axle shafts 5on5 bolt pattern isn't too easy.
Are these normally 31 3/8?

Any assistance is greatly appreciated. I've been researching for days and getting frustrated.

Thanks in advance!