to determine the axle ratio either the thing will have a tag as mentioned above or in most cases they are missing. If missing then rotate one axle one revolution and count the number of turns the pinion makes. 1 tire revolution to 2 3/4 pinion turns its a 2.7x gear etc.
Before installing I recommend inspecting the axle condition if for no other reason than to change the gear lube. To do so, drop the cover off the back of the housing - 10 bolts with a drain pan under it, and a little prying. You will then be able to read the ratio directly off the edge of the ring gear, somewhere along the edge the ratio will be marked, just turn the ring gear until you can see it. Look for excessive metal particles in the oil and on the floor of the housing under the ring gear.
While doing this look at the wear pattern on the ring gear teeth, if its roughly an elongated oval well centered on the gear teeth then its probably okay.
Going further I think its a well thought out plan to inspect the outboard wheel bearings its really easy to do. If you dont have a service manual get one to read up on this.
To do this you need to yank the deathpin lock bolt, and pull the spider gear shaft, once out push the axles in and remove the c clips, and pull the axle shafts part way out. Inspect the bearing surface for damaged areas. If good reinstall them and reverse the whole procedure. If the bearing surface on the axle is damaged you can buy problem solver bearings that move the wear surface away from the damaged part. I think its just good practice to replace the outboard axle bearings on these c lock rearends, so I'd replace them whether the axle shaft has issue or otherwise. But thats me !
I'd also flush the axle housing out with some gasoline just to remove the residual oily crud and then allow it to dry out. Then reassemble the whole mess...
Good luck
