Hopefully you can locate the ratio tag on the rear axle. There may still be the original tag indicating the ratio in place. It could be a 5.38 or a 4.88. The tag was located under one of the rear cover bolts on the axle housing. It was usually the lower right corner but it could be under any one of them. If all else fails just jack it up and put it in neutral. Rotate the driveshaft and count the rotations to make the tire turn one complete revolution.

Here is a good calculator from the tech library on this site: