[Mike C]

If the front axle is original and not messed with, it should have a ratio tag on one of the bolts that will show what the r&p ratio is. If 4x4 works, then the rear will be same. Does it say on the SPID in the glove box what gear is? 3.73 most likely would be original. I'd just swap the disk parts onto the open knuckle if it was me and keep the gears you have. If the '72 axle has a posi on the 3.08, you could always use a ring gear spacer to put the 3.73 on it. I ran a thick 4.10 gear in my Jimmy's 12 bolt on the 3.08 posi for a long time.