how do I figure out

[chesapeake]

What gear ratio this 12 bolt rear has in it ? And as a new to me what should I do to it other than change the fluid?

[R Pope]

Shouldn't need anything done to it, I had one went 540K miles and never touched it.
Hold one axle solid, turn the pinion and count the turns to turn the other axle once. Double that number for the ratio, unless its a posi, then both axles will turn, just count the turns and that's it.

[chesapeake]

Thank you all for your help !