Do your axles have a four bolt flange on them for brake backing plates?
If so, measure from plate to plate in the front and rear and top to bottom, there should be a small variation in the top to bottom, but none from front to rear.....the top measurement should be approximately 1/64" longer than all other measurements while " un-sprung"
When these drop axles are bent, it is typically very difficult to see...
Here is the ultimate way to check your axle condition....
Pull the entire axle out of the spring perches. Then re install the rims/tires and put on a level surface and get a couple of levels and bungee them to the rims (duct tape works too). Now with the levels on each rim, rotate the axle, leaving the tires chocked and note the movement of the rims camber from plumb, there is typically about 2-5 degrees of camber built into the axle for keeping the contact patch of the tire pretty square, this will lesson with a heavy load. If an axle with too much capacity is used, the camber will be off and the tires will wear funny, they will actually smoke the outer edges off...
The levels should be plumb when the axle is at 90 degrees to the ground and have some positive amber when straight up and down....
If not, it is axle time, as was sated, Dexter is a great source, be sure ab buy what you need and don't go "heavier"....
Also note that there is no caster when the trailer is sitting on a level surface, caster is when the axle is actually not perfectly vertical. This will cause an ill handling trailer and cause severe trailer tire problems...
GT
