I haven't heard of this problem before either, however if you decide that it isn't any of the above problems then you may want to re-torque the bolts holding the axles in place. Make sure both sets of bolts are in the same bolt holes as the other side so your axles are straight. Some of those 2 place trailers have an option for a second axle and if the bolts were off a hole that could cause some possible vibration.
Also, we always recommend that you torque your lug nuts after you pull it loaded and empty after a hundred miles or so. Fewer miles if your dealer didn't properly torque them to begin with.
If you maybe hit a curb or something similar that might tweak the axle out of alignment and that may be an issue.
The bearing might not be adjusted properly. You can take off the dust cap and adjust them yourself just make sure they you do not over tighten them, or take the trailer to someone who can adjust them for you. Just make sure that when you spin the wheel to check for adjustment that your brakes, if you have them, are not grabbing and making you think that your bearing are too tight.
If you make it over to Wisconsin, you could run it by the factory and they can take a look at it and see if it is a bad axle. They should be able to tell if the axle is bad and put on a new one for you right away. If you let me know that you are headed that way I'll give them the heads up that you are coming and I'll get someone to take care of you. It is very rare for an axle to be built a little out of wack but it does occasionally happen.
I'll sure help you if I can. I hope my 2 cents helps you out.
Thanks,
Olie