It is, in fact, a bit of both (axle SYSTEM part, technically it's suspension.) The square tube contains the rubber rods that provide the actual suspension and weight-carrying action in the system.
As previously mentioned, some upward curvature near the center of the square tube is normal. The thing to look as is the large metal arm that connects the square tube to the spindle that carries the wheel. With the trailer sitting down on the axles you want to see that arm horizontal or trailing downward a bit (from the square tube down toward the wheel.) If the arm goes up toward the wheel OR if the arm doesn't move when you unweight the system (by jacking the trailer up on the flange where the axle system mounts) then they need to be replaced.
Il Carriaggio — 1975 Argosy 24 | Il Progetto — 1976 Argosy 28 Center Bath | WBCCI# 15566
He has all of the virtues I dislike and none of the vices I admire. — Sir Winston Churchill