Rear Shock Question?

[YourBuddy'sTruck]

I'm about to change the rear shocks in my '70 C20.
I've never done this job before, but after reading about it here, I feel I can easily nick this one myself.
Crawled under to shoot some PB on the bolts and noticed that I have 2 Pin Locations for Shock Mounting?
I'm clueless as to why there would be 2?
One is 16 1/8" from the bottom mount.
The other is 18 1/4" from the bottom mount.

Any ideas as to why there are two?
Would one work better than the other?
Did someone do this just to raise the rear end two inches?

I do haul heavey pallets of products once a week.
Thanks for any advice and/or explanation.