Obviously you have (2) cars the 64” and 72” models is one difference. Next is the fronts are different depending on if you have smart lock or the basic visco lock. On visco driver and passenger side are different, smart lock both sides the same. Rears are same.
I’m not positive if within these differences that they didn’t make improvements like larger diameter on the newer models
smart lock has fronts same both sides, rears same both sides so (2)
non smart lock models have different driver and passenger fronts, rear same both sides so (3)
dont focus on the model XRS, XRS RR, XMR or XRC RR etc...., just look at the dash and see if its smart lock or not.
I had a 17 xrs and it was visco so I carried (3) spare axles in my trailer. I now have a 20 XRS RR with smart lock and I only need (2) one front and 1 rear.
the visco and smart lock fronts are not interchangeable due to lengths. Rears should be same on all 72" models