If you ever see your trailer shaking from an unbalanced tire, it is likely getting a tire separation. Trailer tires are not usually balanced. However they don't shake the trailer when everything is 'normal'.

Talk about "Excessive Road Force Trauma!" The trailer would shake and then the truck would shake, then the trailer would shake... As seen from a spare mounted on the trailer. Separated 2 tires on the front axle.

Made in China Hi-Run trailer tires standard on new Haulmark trailers. Only lasted 2 weeks and 5000 miles. http://www.tbssowners.com/forums/images/smilies/furious.gif Warranty, yes... But a PIA. In fact I have been fighting the dealer/manufacturer/tire reps for 3 months to reimburse me for purchasing 4 new tires to replace their cr@p.

Note the tread being out of round. The blisters inside the tire are scary. The other tire could be felt and wasn't this photo ready.

I had the same issue with my 2007 Haulmark and installed Hi-Run tires. They lasted all of maybe 2,500 miles and had to fight to get them replaced. They were replaced with the same Hi-Run tires that wore out even sooner. Turns out it was due to the axles being setup incorrectly.

You might want to check to make sure the tandem axles are installed parallel and properly positioned to the tongue. Mine were not setup correctly from the factory and it took a year to finally get Haulmark to fix it. Still wears tires unevenly, but not as fast as before.