Sounds like one or more of the axles are not square.
I would measure from a center point on the front of the trailer to the side of each axle near the frame.
If they are not the same measurement the axle (s) are not square. If they are not square with the frame the axles will fight each other loaded or not.
On the bright side the fix should be fairly easy. Cut the spring mounts off the frame, adjust the new mounts for square and weld them on the frame