
I had the same problem on both trailer tires. I pulled the axle to straighten it and I discovered that the spindles, not the axle was (were?) bent.
The guy at the trailer shop told me that it as a very common problem (esp here in Louisiana). Hit a few potholes and they'll bend so slowly you won't notice it until it is too late.
I had them replace the axle (upgraded a ton) and put oil-bath hubs on it. after about 2000 miles, no problems.
Try pulling the wheel and hub off and see if the bearings are eating into the spibdle, it's a dead give-away.
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