Well 2 things come to mind:
1. your brakes are draging and cuasing too much heat. makeing the barring get hot.
2. Your trailer is not level while towing cuasing 2 much weight to be on one axle. ( If I was a betting man, I would put my money on this one.)
Is your trailer leaf spirngs to torsion? Also have you ever checked to see if the trailer is siting level when it its full of gas/ gear and on the back of the truck? This is VERY important if you have torsion axles.
there are a few other things that it "could be",but those are not very commen so your chances of it being a prob are less then 10%.