Jack it up in the back, put it in neutral, then turn the drive shaft until the wheels turn 10 times and count how many times the driveshaft turns (example 32 1/3 means 3.23, which is what I have). This works great if it is a safe-T-trac. If it's open, it gets more complicated.

If it's an original 67 rear it will have traction bar mount(s) on the axle tubes. If not, it was swapped out for another year.