Welcome to the 'bolt Mater. If you can post any casting or stamped numbers on the rear end someone on here will probably be able to identify it for you. Also, do you know what the ratio is now? If you're not sure do this:
Jack up the rear of the truck and put the tranny in neutral. Mark the tire and the driveshaft with chalk or masking tape, or whatever. Rotate the tire 20 times while counting the driveshaft revolutions. Divide the number of driveshaft revolutions by 10 and you've got the ratio (i.e. 41 revolutions would be a 4:10 ratio)
Note, if the rear end is a posi (both rear wheels turn in the same direction as you turn them) just rotate the tire 10 times and divide by 10.
