Look for reinforcement of the main frame rails where the axle assembly is bolted on. I have a Sled Bed aluminum trailer, bought new in November 2001. At the end of the '04/'05 season both frame rails were split where the axle assembly rear mounting bolt goes up through the aluminum frame rail. Up to that point I was very happy with the trailer. It has a wishbone style tounge, a very good lever setup to lock the bed down (much better than the typical pin through a hole style), and it is well balanced.
So if you see the axle just attached with bolts just going through a thin wall aluminum extrusion that has no kind of reinforcement - beware - there is a good chance the aluminum will break there. Fortunately I found my problem while putting the trailer away last spring, and not on the highway after a major failure.
O-Thump