[Axle]

I'd guess different years, different manufactuerrs, different materials. Things I've noticed between my '83 and '81; the 81 plastic is crumbly and brittle. Those inner fender sheilds, rotten on the '81. Come apart in your fingers.
On the '83, the door panels are more flexible, have some sun bleaching but not crumbly (it has been an outside truck 24/7). The '83 inner fender splash shields also, solid, don't tear, and you can pull on em without worry, just torn edges where they rest on the edge of the control arm.