Donna's correct -- Likely the original bushings were relatively thin nylon sleeves and have worn away to nothing. Where the metal is touching other metal, you now have grinding. All this is making slop in the motion and that slop is potentially getting to the tires
and affecting their contact angles with the road.
Were it my trailer, I would renew the shackles, bushings and bolts, plus take a careful look at the springs leaves themselves (many are built with nylon rubbing shims between them), and I would likely use the bushings Donna mentioned. However, unless there is distinct wear on the tires
, you could oil them occasionally and go a long time. BTW, WD-40 is a water-displacing penetrating oil, but not really much of a lubricant, so after using it to loosen stuf, it should be followed up with some grease or lube oil.
Most leaf-spring nuts/bolts that I have seen have special locking threads on them that are essentially destroyed when disassembled, so bear that in mind. Also inspect the hardware holding the axle
beam to the springs in the center of them.
Note that Dexter warrants new torsion axles for five years but only warrants new leaf spring axles for one year.