Check the forum for a link from dremu to his page with all the brochures.
If there is a truck one up (would have to be pickup, not K5) then it will almost certainly tell you what gear was in there. Of course, CA or FED as a destination sometimes matters, as do other things. (transmission for instance)
It is best/easiest to pull both covers off. Most yards I've dealt with will give you a pan for the oil, and let you go out and pull the covers off. 1/2" socket on a ratchet will get the covers off, maybe a flat blade screwdriver to make breaking any seal easier. Will need to get at least one tire on each axle off the ground, if they are still stock, to rotate the carrier enough to get the numbers.
Now you'll know the exact ratio for each axle (stamped on the outside edge of the ring gear) AND the condition of the assemblies in regards to the carrier. There is no telling what has gone on with that truck since it was built, you could find mismatched axles, worn components, gears numerically lower than stock, you name it.
