I've got four of these "holes". One in front and one behind the rear axle on both the right and left leaf spring. I have the composite/fiberglass leafs.
This was in one of the holes. The other three are missing. Do I need them? What are they called and where do I get them?
I believe it's some kind of overload or bump-stop arrangement specific to the fiberglass springs. They're not shown in the diagram, but these instructions for converting the springs from fiberglass to steel include the statement "It is not necessary to re-use rubber pads from OE Composite Spring":
